Let dominant allele be B
Let recessive allele be b
Genotype of male fly = Bb
Genotype of female fly = Bb
Simple monohybrid cross produces 3 genotypes: BB, Bb and bb (You should know how to do the cross).
Number of phenotypes is only 2. BB and Bb produce brown flies whilst bb produces black flies.
Max number of genotypes : Max number of phenotypes = 3 : 2
So C is the answer.