A Haploid yeast strain that is deficient in its ability to make arginine, is crossed to a different haploid yeast strain also unable to produce arginine. The resulting diploid, F1 strain is able to make its own arginine. What is the best explanation?

a. Each deficient haploid strain possessed a mutant allele, but not in the same gene, thus the diploid had 1 functional copy of each gene
b. The defective arginine biosynthetic enzymes, made from the mutant genes, were able to interact and make arginine
c. A crossing over event with one of the yeast strains gave a functional allele of the arginine biosynthesis gene

As it is mentioned in the question itself that each haploid strain are not able to produce arginine, and cross of the haploid strain to other haploid yeast strain produce the yeast offspring that can produce arginine.

This establishes that the mutations in deficient strains are on different genes so the F1 progeny has a copy of each gene is functional so synthesizing arginine is completed.

Thus, the correct answer is option A.