So since there are 4 terms given, pair the first two and second 2.
(x^3 + 2x^2) + (4x + 8) factor out the common terms in both brackets
x^2 (x + 2) + 4 (x + 2) i factored out x^2 from bracket 1 and 4 from bracket 2
Notice that there are two terms left. x^2 (x + 2) and 4 (x + 2)
Also note that both these terms have matching brackets (x+2)
So you can "factor out" x+2 from both terms like this:
(x + 2) (x^2 + 4) < to get the second bracket, i just factored out x+2 and put the remaining x^2 and 4 into one bracket. So your answer is D.