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The key to this question is to carefully think about the first sentence and to apply your understanding of KE and PE to the situation. See A Very Fine Detail section.
Kinetic energy depends upon mass and speed. See Dictionary section. A constant speed means a constant kinetic energy. And any change in speed will lead to a corresponding change in kinetic energy.
Potential energy depends upon mass and height. See Dictionary section. A constant height means a constant potential energy. And any change in height will lead to a corresponding change in potential energy.
The total mechanical energy is the sum of the kinetic and potential energies. See Dictionary section. If either KE or PE increases while the other remains constant, then the sum (total amount of mechanical energy) will obviously increase. If either KE or PE decreases while the other remains constant, then the sum (total amount of mechanical energy) will obviously decrease.
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There are some details in the first sentence that serve as clues to the answer. In the many similar questions on this sublevel, you will have to give attention to these clues. In this question, the clues are "speeds up" and "moving horizontally". The "speeds up" clue indicates an increase in speed. The "moving horizontally" clue indicates a constant height.
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Definition of Kinetic Energy:
Kinetic energy is the energy possessed by an object due to its motion. If an object is moving, then it has kinetic energy. The amount of kinetic energy depends on mass and speed.
Definition of Potential Energy:
Potential energy is the energy stored in an object due to its position. The most common type of potential energy - gravitational potential energy - is the energy stored in an object due to its vertical position relative to the ground or some zero level. The amount of gravitational potential energy depends upon mass and vertical height.
Definition of Total Mechanical Energy:
The total mechanical energy possessed by an object is the sum of its kinetic energy and potential energy.
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