Answer
Option A is correct.
Proving that ABD is congruent to CDB proves that the 'opposite sides of a parallelogram are congruent'.
Explanation
To prove that two triangles are congruent, we must have established that the two triangles have sides that are congruent. And the sides of triangle ABD and triangle CDB are coincidentally the sides of the parallelogram in question.
So, that these sides are congruent shows that the opposite sides of the parallelogram are congruent.
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