Answer:
It is C.
III is saying that the two functions are inverse functions. If two functions, f(x) and g(x) are inverse, then f(g(x)) = x and g(f(x)) = x. Therefore, we see that if III is true, then I and II should also be true. Now, all we have to prove is that f(x) and g(x) are inverse functions. As we said before, if two functions are inverse, then f(g(x)) = x and g(f(x)) = x.
f(g(x)) = 3()^3 + 2
f(g(x)) = 3[(x-2)/3]+2
f(g(x)) = x-2 + 2
f(g(x)) = x
Therefore, we have proved that f(g(x)) = x, and so f(x) and g(x) are inverse functions, and so all of I, II, and III are true. Thus, the answer is C.
fyi, we didn't have to do g(f(x)) because if f(g(x)) = x, then so will g(f(x)).