Answer:
Converse of the Alternate Exterior Angles Theorem.
Explanation:
The alternate exterior angles theorem explains that "if a pair of parallel lines are cut by a transversal, then the alternate exterior angles are congruent." Judging from the fact that the problem is including <1 and <8, we can safely assume that it is including this theorem since both of said angles are on the exteriors of the pair of parallel lines. Not to mention that this principle also effectively proves that both lines are parallel to each other, thus proving this sentence true.