Concept: The question tests on Functions 


g(f(x)) means g of f(x)

For example g(f(2)) = g(2222 = g(4) = 7. from this itself, we see that if we put x = 2, Option D gives us 7.

We can also get the actual value of g(f(x)) by putting x = x2x2 in x + 3

Therefore g(f(x)) = x2+3x2+3

Option D