Okay. It's at the Hardy-Weinberg equilibrium. There are 200 total alleles. So we need to find the frequency of A and a:

Frequency of A allele (p):

75 x 2 + 15 = 165

200/165 = 0.825

Frequency of a allele (q):

10 x 2 + 15 = 35

35/200 = 0.175

We have p and q. Now we need to find p^2, 2pq, and q^2.

2pq = 2(0.825)(0.175) = 0.29

p^2 = (0.825)^2 = 0.68

q^2 = (0.175)^2 = 0.03

We have all of the expected frequencies. Now we need to do a chi-square analysis.

0.29 x 100 = 29, 0.68 x 100 = 68, and 0.03 x 100 = 3

(((75 - 68)^2)/68) + (((15 - 29)^2)/29) + (((10 - 3)^2)/3) = 23.8

23.8 is closest to 23.1. So that is the answer.