Final answer:
The fraction of the population that is heterozygous for alleles A and a in a Hardy-Weinberg equilibrium population, given that p (the frequency of A) is 0.4, is calculated as 2pq, which equals 2*(0.4)*(0.6) or 0.48. So the correcct option is D.
Step-by-step explanation:
In a population that is in Hardy-Weinberg equilibrium, the frequency of two alleles, A and a, are represented by p and q respectively, where p + q = 1. When given that the frequency of allele A (p) is 0.4, we can calculate the frequency of the allele a (q) as 1 - p which equals 0.6. To find the fraction of the population that is heterozygous (Aa), we use the 2pq part of the Hardy-Weinberg equation, which in this case is 2*(0.4)*(0.6), leading to an answer of 0.48.