You are correct. C is correct answer, but because of same explanation, B should also be correct answer.
gof(x) = g(f(x))
On computing f(x), it gives output that belongs to the set T. This output serves as input of g(x). But the valid domain set for g(x) is S, while it is receiving its input in domain T, hence we say that gof(x) is not defined too.