Answer is B.
You can do back substituting, $T(n) = T(n^{1/2}) + 1 = (T(n^{1/4})+1) + 1 = ((T(n^{1/8})+1)+1) + 1$
In general, we will get, $T(n)= T(n^{\frac{1}{2^k}}) + k$
Now we will back substitute till the recursive term is eliminated, assuming T(2) to be const.
i.e. $n^{\frac{1}{2^k}} = 2$, take log on both sides, $2^k = log n$, again take log on both sides, and you will get $k = log log n$
thus $T(n)= O(log log n)$