Using, AD as a transversal, ∠A and ∠ are same-side interior angles, so they are . Using side as a transversal, ∠B and ∠C are same-side interior angles, so they are supplementary. Using AB as a transversal, ∠A and ∠B are same-side interior angles, so they are supplementary.

The side that connects vertices B and C is BC, so it acts as the transversal for ∠B and ∠C. These angles are same-side interior angles formed where BC crosses the parallel sides AB and CD, so they are supplementary. The correct choice is BC.

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