Geometry

Question Analysis：
This problem requires identifying which additional angle measurement would complete the parallelogram proof.

Key Concept Explanation：
To confirm a parallelogram：
• Need both pairs of adjacent angles to be supplementary
• Given one pair (∠P + ∠Q = 180°), must confirm the other pair

Step-by-Step Solution：
Already know ∠P + ∠Q = 180° (satisfies one adjacent pair)
Need confirmation that ∠Q + ∠R = 180° or ∠P + ∠S = 180° (the other adjacent pair)
This would force：
• ∠R = 80° (opposite ∠P)
• ∠S = 100° (opposite ∠Q)
All parallelogram angle conditions would then be satisfied

Option Analysis：
• A) Only shows one opposite angle pair and ∠R = 80°
• B) Only shows one opposite angle pair and ∠S = 100°
• C) Correct - confirm...