A block slides across a flat, horizontal surface to the right. For each choice, the arrows represent velocity vectors of the block at successive intervals of time. Which of the following diagrams represents the situation in which the block loses kinetic energy?

The diagrams the question refers to is in the attachment.

Third diagram or alternative C.

Explanation: In physics, vector is a representation of magnitude and direction.

Kinetic Energy is energy made by a moving object, i.e., when an object with mass has velocity, it also has Kinetic Energy. It's calculated as:

Vectors below are representing change in velocity during intervals of time.

In the first diagram, vectors doesn't change, so velocity and kinetic energy are constant.

At the second, velocity still constant, however, the "intensity" of the arrow is weaker than the first, which means velocity is less than the first diagram and so is its kinetic energy.

In the fourth diagram, the arrows grow bigger from left to right. This means velocity increases during time. If velocity grows bigger, so does kinetic energy.

In the third diagram, the arrows decrease as it goes to the right: the block's velocity decreases during the successive intervals, which means, kinetic energy diminishes through the interval.

Thus, diagram C represents the situation in which the block loses kinetic energy.