Thanks Guys. The answer is 'E'.
The way I solved it was
a. Square root of 4 is 2
b. cubeth root and 4th root of 4 should be greater than 1. So we have alteast 4.
However, I was wondering if there is any formula that I can use, because it were not 4 say 9 or 13 or something, then I may not be able to do the cubeth root or fourth root.
Good is the greatest enemy of great.