The acceleration of a projectile at the top of its trajectory, when its instantaneous vertical velocity is zero, is still g, the acceleration due to gravity.

Here's why:

* Projectile motion: A projectile's motion is governed by gravity, which acts constantly and vertically downwards.

* Vertical velocity: While the vertical velocity becomes zero momentarily at the peak of the trajectory, the projectile is still under the influence of gravity.

* Constant acceleration: Gravity causes a constant downward acceleration, regardless of the projectile's instantaneous vertical velocity. This means even at the top, the projectile is still accelerating downwards at approximately 9.8 m/s².

Important Note: While the vertical velocity is zero at the top, the projectile still has horizontal velocity, meaning it continues to move horizontally.