Final answer:
A test cross involves crossing an individual with a dominant phenotype, but unknown genotype, with an individual that is homozygous recessive for the trait to determine the genotype of the dominant individual.
Step-by-step explanation:
A test cross involves crossing an individual with a dominant phenotype, but unknown genotype, with an individual that is homozygous recessive for the trait. This cross helps determine if the dominant individual is homozygous dominant (AA) or heterozygous (Aa).
From the given options, the correct example of a test cross is A_bb X aabb. Here, the dominant individual has a dominant phenotype (A_), but its genotype is unknown. By crossing it with an individual that is homozygous recessive (aabb), we can determine the genotype of the dominant individual.