Answer:
72 is your answer mate...
S^2 is the only denominator. It is the least common denominator.
Well if the original population is 250 crickets after 6 months it would double to 500. After another 6 months (A year now) it would again double in size to 1000 and so on for 2 more periods of 6 months. After doubling two more times you get 4000, or answer D.