Question

When is a rectangle a regular polygon?

Answer 1

A rectangle is never a regular polygon.

Explanation:

A regular polygon must be both equilateral (all sides are the same length) and equiangular (all angles of the same measure). A rectangle is equiangular. All of its sides equal 90˚ by definition.
However, a rectangle can never, by definition, be equilateral. All of its sides can never be the same length and so, it can never be regular.