Pretty sure it’s
C. Heterozygous advantage.
Chart shows that as fitness increases (as you look from left to right on the x axis), the prevalence heterozygous alleles increase, and prevalence of homozygous decreases. I mean you can see that the homozygous is concentration at the start, at the far left side of the plot. If you look at the x axis where it starts, the x axis tells you that fitness is lower there. This shows that heterozygous alleles are dominating in this population when it comes to fitness.