Human abo blood groups are determined by a single gene with 3 alleles: a, b, and o. in a sample of 300 individuals, 100 are blood type a and genotype aa, 100 are blood type b and genotype bo, and 100 are blood type o and genotype oo. what are the allele frequencies

Allele frequency is determined by looking at the dominant and recessive allele of a genotype. 
Using the given, we can come up with the following computation for the allele frequency:
allele A = 100/300 allele B = 50/300 allele O = 150/300
Hence, the frequency of allele A is 0.33 or 33%, allele B is .17 or 17% and allele O is 0.50 or 50%.  

The correct answer is c.