A function of x means that you get an answer if you plug in a certain value for x.
A is not a function, because it specifies that x=5
B could be a function, but you'd have to rewrite it in a form y = ....
C is a nice function of x. Just swap it: y=x2
D is a circle. You can't rewrite it as a clean function of x.
So I would choose C.
