Evolution: Mutation and Selection?

The answer is D. because (1) for an organism to have 100% fitness it must be heterozygous at many loci, and (2) if it is heterozygous at many loci, it will produce at least two kinds of gametes, (3) the actual number of different kinds of gametes will be much greater than two because of crossing-over, and (4) zygotes produced from different kinds of gametes will necessarily differ in fitness.