"Stroboscopic photography" is a commonly used experimental method to study the motion of objects, such as stroboscopic photos taken by stroboscopic cameras when different objects are moving.

First, the object passes through the same distance at the same time, so the object moves in a straight line at a uniform speed and is in a state of balance, so the object is subjected to a balancing force. So a is correct;

B, the distance that the object passes in the same time decreases, so the object slows down and is not in equilibrium, so it is not affected by the balance force. So, B is wrong;

C, the distance traveled by an object in the same time increases, so the object is accelerating and not in equilibrium, so it is not affected by the balance force. So, c is wrong;

D, the object in the same time through the same distance, but its trajectory is curved, so the object is not in a state of balance, so it is not affected by the balance force. So, d is wrong.

So choose a.