Answer:
c. A testcross
Explanation:
A dominant organism can be heterozygous or homozygous for the gene. To determine the genotype of a dominant organism, it is crossed with a homozygous recessive organism for the same gene. In the given cross, a tall pea plant with an unknown genotype is crossed with a pure breeding short pea plant. This is called a testcross.
If the obtained progeny express both tall and short phenotypes, the tall parent plant was heterozygous (Tt) for the trait. If the cross gives only tall progeny, the genotype of the tall plant is homozygous dominant (TT). Here, T represents the allele for tallness while the allele "t" gives short phenotype.
