Explanation

Try the form (√m − √n)² = (m+n) − 2√(mn) = 15 − 4√14 = 15 − 2√56. Then m + n = 15 and mn = 56, giving {m, n} = {8, 7}. So the square root is √8 − √7 = 2√2 − √7. Hence option A.