Which statement is true about acceleration?

a. it is the rate of change of speed per unit time
b. it is the rate of change of displacement per unit time
c. it is the rate of change of velocity per unit time
d. it is the rate of change of position per unit time

Acceleration is always the rate of change of velocity per unit time.  ( C. )

Notice that a change of direction is a change of velocity,
even if speed doesn't change.

This means that acceleration can take place even if speed isn't changing.

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.