f ∘ g means f(g(x)). So where we have x in f(x) we put g(x): -2g(x)+5.
Next we replace g(x) in this expression with 3x²: -6x²+5.
But this is not one of the answer options, so there is an error in the question. Please check the definitions of f(x) and g(x) from your text book or teacher. I think it’s option c, but there’s a minus sign missing.