The correct answer is C. Testcross.
A testcross is a cross between an individual with an unknown genotype and an individual with a known recessive genotype. The results of a testcross can be used to determine the genotype of the individual with the unknown genotype.
A backcross is a cross between an individual with a dominant phenotype and an individual with a known recessive genotype. The results of a backcross can be used to determine the genotype of the individual with the dominant phenotype.
A dihybrid cross is a cross between two individuals that are heterozygous for two different genes. The results of a dihybrid cross can be used to determine the genotypes of the parents and the offspring.
Pedigree analysis is a method of tracking the inheritance of a trait through a family tree. Pedigree analysis can be used to determine the genotypes of individuals in a family and to identify the mode of inheritance of a trait.
In the case of a plant showing a dominant phenotype, a testcross can be used to determine the genotype of the plant. The plant is crossed with a plant that is homozygous recessive for the trait. If the plant produces offspring with the dominant phenotype, then the plant is heterozygous for the trait. If the plant produces offspring with the recessive phenotype, then the plant is homozygous dominant for the trait.
A backcross would not be useful in this case because the results of a backcross would not be able to distinguish between a heterozygous plant and a homozygous dominant plant. A dihybrid cross would not be useful in this case because the plant is only showing a single trait. Pedigree analysis would not be useful in this case because the plant is not part of a family.