In a test cross, a pure-breeding plant is crossed with a plant suspected to be heterozygous (Aa). What is the genotype of the pure-breeding plant?

aa

Explanation:

In genetics, a test cross is a cross performed to determine the unknown genotype of a phenotypically dominant organism. The test cross is done by crossing the unknown genotype with a homzygous recessive organism. The result of this cross tell us whether the unknown genotype is heterozygous or homzygous dominant.

According to this question, a test cross was conducted involving a pure breeding plant with a plant suspected to be heterozygous (Aa). Based on the explanation of a test cross above, the genotype of the pure-breeding plant will be "aa". Note; it must be recessive.