(D) 9 cm.
Parallelogram:
- A simple (non-self-intersecting) quadrilateral with two sets of parallel sides is known as a parallelogram in Euclidean geometry.
- A parallelogram's facing or opposing sides are of equal length, and its opposing angles are of similar size.
- The Euclidean parallel postulate or one of its equivalent formulations must be used in order to demonstrate the congruence of opposed sides and opposite angles because both conditions are a direct result of this postulate.
- In contrast, a quadrilateral with only one set of parallel sides is referred to as a trapezoid or trapezium in British or American English.
- The parallelepiped is a parallelogram's three-dimensional equivalent.
Therefore, the correct answer is (D) 9 cm.
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