Answer:
sqrt(46)
Explanation:
D is almost 7 (just a little bit below).
the square of 7 is 49, so D must be the square root of a number a little bit lower than 49.
it can't be the cubic root of 50, because we are looking for the square root of something close to 49.
50 is close, but the cubic root makes it much, much smaller.
the cubic root of 130 is close to 5 (as 5³ = 125, so, close to 130), so it is also too small to be D.
and the square root of 39 is much closer to 6 than to 7.
so, that leaves us with sqrt(46), which is indeed close to 7 (as 46 is close to 49).