Which of the following pairs of line segments is perpendicular to a rectangle?

In a rectangle:

A. Opposite sides: These are parallel (not perpendicular) because rectangles have opposite sides that are parallel and equal in length.
B. Adjacent sides: These are perpendicular because rectangles have adjacent sides that form right angles with each other.
C. Diagonals: The diagonals of a rectangle bisect each other but are not necessarily perpendicular. They only form right angles if the rectangle is a square.
D. Parallel sides: This is not a specific category; parallelism applies to both opposite and adjacent sides.

So, the correct answer is Adjacent sides.