If a baseball has zero velocity at some instant, is the acceleration of the baseball necessarily zero at that instant? Explain, and give examples.

No, not necessarily

Explanation:

If a particle moves with an acceleration that induces its velocity to be reduced, then there will be a moment when it reaches v = 0, but this does not automatically mean that the acceleration will no longer act. When, the body resumes its motion, with the same acceleration, then velocity is negative.

For example when ball thrown from the ground reaches it's maximum height, it that instant it's velocity is zero but acceleration is g m/s^2 acting downward