Here's how you can simplify the equation:
First off we need to simplify √12x^8, and write it as (√3x^2)(√4x^6),
We could then find the answer:
Thus the answer is 2x^3.
I notice that one of your answers (C,D) are repeated,perhaps you typed something wrong? The answer is either C or D in that case,do tell me if you cannot find 2x^3 at all tho.
Hope it helps!