In garden peas, the axial (A) flower position is completely dominant to the terminal (a) flower position. You cross a true-breeding axial flowering plant to a true-breeding terminal flowering plant. What is the genotype of the true-breeding axial flowering plant?

AA

Explanation:

A true breeding organism for a particular trait is an organism that would produce progeny  with the same trait whe self fertilized.

Hence, since axial flower is represented by the allele A, a true breeding flowering plant will have the genotype AA.

Axial flower (A) is dominant over terminal flower (a).

True-breeding axial flowering plant will have the genotype AA.

True breeding terminal flowering plant will have the genotype aa.

AA is crossed with aa.

AA   x   aa

offspring: Aa, Aa, Aa and Aa.

All the offspring will exhibit axial flowering.