If the velocity of a body is halved then its kinetic energy will:

A particle or an item that is in motion possesses a sort of energy called kinetic energy. An item accumulates kinetic energy when work, which involves the transfer of energy, is done on it by exerting a net force.

 Kinetic Energy $=\frac{1}{2}m{v}^2$ ( $m$ is the mass of the body and $V$ is the velocity at which the body is moving)

Then, the equation changes when the velocity is cut in half.

Kinetic Energy $=\frac{1}{2}m{v}^2=\frac{1}{2}m {\left(\frac{v}{2}\right)}^2=\frac{1}{2}m\frac{v^2}{4}=\frac{1}{4}\left(\frac{1}{2}m{v}^2\right)$

We can see from the equation above that the kinetic energy has decreased to one-fourth of its initial value.