The Hardy-Weinberg equation is as follows:
p² + 2pq + q² = 1
Where p is the frequency of the homozygous dominant genotype, q is the frequency of the homozygous recessive genotype and 2pq is the frequency of the heterozygous genotype. Using the 2pq term here,
Frequency of heterozygous = 2 * 0.7 * 0.3
Therefore, option C is correct.