The acceleration of the stone at the top of its trajectory is 9.8 m/s² downwards, which is the acceleration due to gravity (g).

Here's why:

* Gravity is constant: Regardless of the stone's motion, gravity constantly pulls it downwards.

* At the top: At the very peak of its trajectory, the stone momentarily has zero vertical velocity. However, it's still under the influence of gravity.

* Acceleration and Velocity: Acceleration is the rate of change of velocity. Even though the stone's velocity is zero at the top, gravity is still causing it to accelerate downwards, preparing it to fall back down.

Let me know if you'd like to explore other aspects of projectile motion!