Which example identifies a change in motion that produces acceleration?
a. a ball moving at a constant speed around a circular track
b. a particle moving in a vacuum at constant velocity
c. a speed skater moving at a constant speed on a straight track
d. a vehicle moving down the street at a steady speed?

The answer is the option a. a. a ball moving at a constant speed around a circular track.

Acceleration is the change in velocity. This change may be either on speed or on direction (or both).

A ball moving at a constant speed around a circular track is continously changing its direction so it is under acceleration (centripetal acceleration).

All the other cases are of objects moving at the constant speed and in straight line, i.e. constant velocity, which is not accelerating.

  concave means having an outline or surface that curves inward like the interior of a circle or sphere. so the mirror curves inward.