21 is made up of factors 3 and 7. To make this into a perfect square, which would simplify the square root we would need to find a number for x containing an odd power of 3 (3, 27, etc.) or an odd power of 7. The best answer is 54=3^3*2. 21*54=3^4*2*7. So the square root is 9sqrt(14). Option D is the answer.