A parallelogram is a quadrilateral in which:

Correct option is C

Given:
A parallelogram is a special type of quadrilateral.
Formula Used / Definition Used:
A parallelogram is defined as a quadrilateral in which both pairs of opposite sides are parallel and equal in length.
Solution:
Option A is incorrect because in a parallelogram both pairs of opposite sides are parallel, not just one pair.
Option B describes a rectangle, not a general parallelogram.
Option D is a property of a rhombus or kite, not of every parallelogram.
Option C correctly states the defining property of a parallelogram.