Test cross in plants or in Drosophila involves crossing between two genotypes with recessive trait, between two F1 hybrids, the F1 hybrid with a double recessive genotype, or between two genotypes with dominant trait?

A test cross is a test used to determine the genotype of an organism showing dominant phenotype. It was introduced by Gregor Mendel. In this, the F1 hybrid is crossed with a parent having a double recessive genotype. If the unknown genotype is heterozygous, the recessive trait will be present in half of the offspring. However, if the unknown genotype is homozygous, the dominant trait will be present in all the offspring. Thus, the correct answer is option A (The F1 hybrid with a double recessive genotype).