B) The square root of .21
The Hardy-Weinberg equation describes genotype frequencies in a population where p^2 is the proportion of the population that is homozygous dominant, 2pq is the proportion of the population that is heterozygous, and q^2 is the proportion of the population that is homozygous recessive.
Since q^2 = 0.21,
q (the frequency of the recessive allele) will be the square root of 0.21.
Hope this helps :)