PussInBoots has a great solution above, and it's particularly useful to notice that you can compute 24^2 - 25*23 quickly using the difference of squares. Considering the answer choices here, however, if you notice that 5*sqrt(23) is almost equal to 24, then sqrt(24 - 5*sqrt(23)) must be very close to zero, while sqrt(24 + 5*sqrt(23)) must be very close in value to sqrt(48) ~ 7. So we're looking for an answer that is approximately equal to 7, and there's only one answer choice that's even remotely close.