Answer
b. genotype
Work Step by Step
In a testcross, an individual who displays a dominant phenotype but whose genotype is not known is crossed with a known homozygous recessive individual. This can determine whether or not the individual whose genotype is unknown is homozygous dominant or heterozygous. If all of the offspring end up displaying the dominant trait, we know that the individual being tested is homozygous dominant; on the other hand, if some of the offspring display the recessive trait, then the individual who is tested is heterozygous.
