A stone is thrown vertically upward with a speed of 15m/s.what is the acceleration of the stone in its highest point of its motion?

Answer 1

The acceleration is
a = -g = -9.8 m/s^2
at all times the stone is in the air.

It does not depend upon the initial thrown velocity.

The minus sign is there because the acceleration is in a downward direction.

Answer 2

Acceleration is "g" at each point of the trajectory