Its A because whenever you see f(x), whatever is in the parenthesis is substituted for x. So if it were f(x)= x and you had to find f(3) then you would plug in the three for x then f(3)=3. The same goes for this. You plug in the whole equation of f(x) into the x in g(x). Since g(x) is just x^2 then you plug in the x-5 and that would be g(f(x))=(x-5)^2. I hope that helped I'm not very good at explaining.
