For a body projected at an angle with the horizontal from the...

In a projectile motion, a particle has two components of velocity. The horizontal component of velocity remains constant over time, while the vertical component of velocity changes.

At highest point, the vertical component of velocity $\left({v}_{y}\right)$ becomes zero, but the horizontal component is non-zero. So the kinetic energy of the particle is not zero.

The formula to calculate the potential energy $\left(U\right)$ of the particle is given by

$U = mgh . . . . . . . . . . . . . . . . . . . ( 1 )$

where, $m$ is the mass, $g$ is the acceleration due to gravity and $h$ is the vertical height attained by the object.

Hence, the potential energy is maximum when the projectile attains the maximum height.