You can eliminate useless answers based on their wording. "Similar" anything cannot be used to prove congruence. "Rectangles" will not help you prove congruence of parallelogram sides. The only reasonable choice is ...
congruent triangles
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Typically, a diagonal is drawn through the figure, and the two triangles created are shown to be congruent, often by ASA. Then, by CPCTC, sides of the parallelogram are shown to be congruent.
