Final answer:
The reason why lines p and q are parallel when x=y is the Alternate Exterior Angles Converse Theorem.
Step-by-step explanation:
The correct answer is c) Alternate exterior angles converse theorem.
When it is given that x=y, it implies that the lines p and q are parallel. The theorem that establishes this relationship is the Alternate Exterior Angles Converse Theorem. According to this theorem, if two lines are cut by a transversal and the alternate exterior angles are congruent, then the lines are parallel.