H(x) = f(g(x)), so the answer is C. Why? g(x) = x^2 + 15. This is what appears under the square root sign in H(x). So, all that's left to do is to take the square root of g(x). What function f does that? f(x) = sqrt(x). So, that answer is the you get H by first applying g to x, then applying f to that.