A rectangle is symmetrical about the lines joining the ---- of the opposite sides A. midpoints B. vertices C. points D. none of the above

To solve this question, let's first understand the symmetry of a rectangle.

A rectangle has four sides, and symmetry in a rectangle refers to how one half of the rectangle mirrors the other half. A rectangle is symmetrical about two specific lines:

The line joining the midpoints of opposite sides (this divides the rectangle into two equal parts along its length).
The line joining the midpoints of opposite sides, dividing it along its width.
When a rectangle is symmetrical about these lines, it means that if you fold the rectangle along these lines, the two halves will perfectly overlap.

Therefore, the correct answer is:

A. midpoints