Final answer:
The correct option for a test cross is to mate the unknown genotype with a homozygous recessive individual. This reveals whether the unknown genotype is homozygous dominant or heterozygous based on the phenotypes of the offspring.
Step-by-step explanation:
A test cross is a genetic cross performed to determine the genotype of an individual expressing a dominant trait, but with an unknown genotype. To achieve this, the individual is crossed with another individual that is homozygous recessive for the trait in question. The resulting offspring's phenotypes can reveal the unknown genotype of the dominant-expressing parent. If all of the offspring display the dominant trait, the unknown parent is likely homozygous dominant. However, if a 1:1 ratio of dominant to recessive phenotypes is observed in the offspring, the unknown parent is heterozygous.
The final answer to the student's question is C. The individual with an unknown genotype is crossed with an individual that is homozygous recessive. This method, established by Mendel, allows the deduction of the unknown genotype by analyzing the expressed traits in the offspring.