Answer:
(C) Alternate interior angles are congruent.
Explanation:
Given: Lines A and B are parallel lines and C and D are transversals.
To prove:

Proof:
Statement Reason
1. Lines A and B are parallel lines; Given
C and D are transversals
2.∠Y≅∠4 Alternate interior angles
3.∠Z≅∠6 Alternate interior angles
4.∠X+∠4+∠6=180° Straight line property
5. ∠X+∠Y+∠Z=180° Substitution property
Hence proved.
Therefore, option C is correct.