Objects that move with a constant velocity (same speed ... same direction) experience a balance of forces. Oppositely-directed forces are of the same strength. On the other hand, objects that accelerate experience an unbalanced force that is directed in the direction of the object's acceleration.
 

The Force Diagram provided for the car shows unbalanced forces: there is more rightward force than leftward force. This will always be the case for an  object that accelerates to the right, such as the case of this rightward-moving car that is speeding up. But you need to determine what the Force Diagram looks like if the same car moves to the right at a constant speed. To do so, you should first convince yourself that this motion would be characterized by a balance of force. Oppositely-directed forces will be of the same strength; so oppositely-directed arrows will be of the same length. This will allow you to rule out some of the provided answer options.
 

What's the Applied Force?

We Physics types like to give names to forces. Descriptions would be a whole lot more meaningful but they are a little difficult to include on these small diagrams. So what description can be given to the applied force (F_app) in the given diagram and what will happen to it when the driver lightens up on the gas pedal? Great questions!  Both of them. Buckle up and get ready for a driving lesson ... Physics style.

Pushing down on the gas pedal causes a car axle (to which the wheels are attached), to spin more rapidly. This causes the wheels to spin more rapidly. And provided there's no ice on the road, these wheels will grip the road and push the road backwards with greater force. And the result is the road pushes the wheels forward with greater force. The forward force is the force of the road pushing forward on the car due to friction between the wheels and the road surface. That's the description. As for the name and label, we tend to call it an applied force (F_app) since it is the force applied by the road on the car. It could just as easily be called a friction force (F_frict). But the important thing is to recognize that this force is the result of pressing down on the gas pedal. And the more that you press down, the more of this force you will get; and the less that you press, the less of this force that you are going to get. 

 

Why Do Cars Accelerate?

As long as we're talking about driving, let's address the question of why do car's accelerate? One might quickly answer that cars accelerate when the driver pushes down on the accelerator pedal (a.k.a., gas pedal). But that doesn't explain why cars accelerate; that eplains when cars accelerate. The why question has to do with forces and the imbalance that results between opposing forces. The previous paragraph discussed the forward force - the applied force (F_app).  Now we will discuss the air resistance force (F_air).

The air resistance force (F_air) is the force that results from high speed objects plowing through the air. The air particles don't get out of the way of the moving object in a gentlemanly fashion. They provide some resistance. As the car pushes against the air, the air pushes back a bit before finally getting out of the way and streamlining around to the back side of the car. This pushing back of the air on the car is called air resistance and it is the cause of the leftward force. Its strength depends on a variety of factors but one of them is the speed of the car. Faster cars experience more air resistance.

Now to accelerate forward, the driver must have more forward force - applied force (F_app) - than backward force - air resistance force (F_air). That's where the gas pedal comes in. Pressing down on the gas pedal increases the spin rate of the axle and provides more forward applied force. Because it is greater than the strength of the air resistance force (F_air), the car accelerates in the forward direction. And now you know why cars accelerate. But what happens when there is a ...  (Read on.)
 

Letting Up On the Gas

When the driver lets up on the gas, the applied force changes. There is still an applied force as long as the car axle is spinning and the wheels are gripping the road.  But if you have followed the discussion in the previous paragraphs, you will understand that letting up on the gas will cause a decrease in the rightward applied force (F_app). And this decrease results in a constant speed motion as its strength decreases to the strength of the leftward air resistance force (F_air). 
 

Think Interactions ... Always!

Forces result from mutual interactions between two objects. To determine the forces that act upon the car, ask what objects are the car interacting with? The car (as always) interacts with the Earth through gravitational forces between the crate the Earth. The car pulls up on the Earth and the Earth pulls down on the car. This is the gravity force (F_grav). But the car also interacts with the road. It presses down on the road and the road presses or pushes up on the car. This is the upward normal force (F_norm).  But the car inteacts with the road in another way: as the car's axles turn and as the wheels grip the road, the wheels push back on the road and the road pushes forward on the wheels. This is the rightward applied force (F_app). Finally, the car is moving through the air and interacts with air particles that are in its path. It pushes rightward on these air particles and the air particles push leftward on the car. This is the leftward air resistance force (F_air).