Inverse variations are of the form x*y = k where k is some fixed number.
We have x = 9 and y = 2.5 so
x*y = 9*2.5 = 22.5
Therefore, x*y = k updates to x*y = 22.5, implying that k = 22.5
This is why the answer is actually choice B
Side Note: We can solve for y to get y = 22.5/x but this isn't listed as an answer choice