C

Step 1: In projectile motion, the velocity vector changes in direction and magnitude under the influence of gravity, which provides a constant acceleration vector downwards.
Step 2: At the topmost point of the trajectory, the vertical component of the velocity becomes zero while the horizontal component remains unchanged. Thus, the velocity vector is entirely horizontal at that instant.
Step 3: Since the acceleration due to gravity is acting downwards (vertical direction) and the velocity is horizontal at the peak, the two vectors are perpendicular to each other at this point.
Step 4: At other points in the trajectory (either at launch or descent), the velocity does not align perpendicularly with the downward acceleration.
Therefore, the correct answer is the topmost point.