(n-1)(n+1) is the same as n(sqrd)-1(sqrd)

(1) this means that n is odd, take 7 for example, 7sqrd - 1 = 49 -1 = 48. when 48 is divided by 24 the remainder is 0. This goes on for all odd numbers except 3. 3sqrd -1 = 8, that leave a remainder of 8
(2) alone this is insufficient

If we combine both, you see that all odd numbers squared - 1 are divisible by 24, answer = C

I hope I make sense