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

Now Alex, enough already, draw picture with all forces.
There is one, the weight
F = - m g
so
F = m a = - m g
so
a = -g

THE WHOLE TIME, NOT JUST AT TOP

Now no more questions until you try yourself !!!!