Since you cannot add numbers which have x to the power of something and normal x, A and B are incorrect. C is also incorrect because I don't even know how you could get that number.
D is correct.
(14x2 + 7x) + 9x2 - you don't even need the brackets, so
14x2 + 7x + 9x2
(14x2 + 9x2) + 7x
23x2 + 7x