Which of these statements correctly describe the formula $S = 2\pi rh$ ?
$S$ is jointly proportional to $r$ and $h$
$S$ is directly proportional to $r$ and $h$
$S$ varies directly with $r$ and $h.$
I am leaning towards choice 3, but am not sure.
This stuff on variation that might help ($k$ is a constant):
"S varies as x" means S = kx
"S varies jointly as x and y" means S = kxy
"S varies as x + y" means S = k(x + y)
"S varies inversely as x" means S = k/x.
The verb "describe" in the question is in the plural. That would seem to imply that any number of answers can be selected. As Chris noted in the comments, all three sentences are OK (assuming an appropriate definition of the terms used).