For general projectile motion, when the projectile is at the highest point of its trajectory, its acceleration is zero? a)the horizontal and vertical components of its velocity are zero. b) its velocity is perpendicular to the acceleration. c)the horizontal component of its velocity is zero. d)its velocity and acceleration are both zero.

B) Its velocity is perpendicular to the acceleration.

Explanation:

For general projectile motion, the horizontal acceleration is 0 and the vertical acceleration is -g.  This is true for all points on the trajectory.

At the highest point, the vertical velocity is 0.  So you have only a horizontal velocity as well as a vertical acceleration.  So the two are perpendicular.