Question

A stone is thrown vertically upwards, reaches a highest point, and returns to the ground. When the stone is at the top of its path, its acceleration __________.

Answer 1

From the time the stone leaves the thrower's hand until it hits the ground, its acceleration is 9.8 m/s^2 downward. Or whatever the acceleration of gravity is on the planet where the toss takes place. It doesn't even have to be tossed vertically. It can be tossed horizontally, or down, or slanty. The answer is the same.