Answer
Volume is doubled.
Work Step by Step
Let $V$ be the volume of a cylinder.
Let $h$ be the height of a cylinder.
Let $r$ be the radius of a cylinder.
$V=khr^2$
When height is halved and radius is doubled,
$k\frac{h}{2}(2r)^2=2khr^2$
Since $khr^2=V$, $2khr^2=2V$
Volume is doubled.
