C. Opposite angles of a parallelogram are congruent.
Angle 1 is congruent to angle 2 because they are opposite angles of the larger parallelogram. Angle 2 is congruent to angle 3 because they are opposite angles of the smaller parallelogram.
The answer to the proof question is C: Opposite angles of a parallelogram are congruent. This is due to the property that states that opposite angles in a parallelogram are equal to each other. This understanding is crucial for establishing angle relationships in the proof.
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