Answer:
A. When its sides are congruent
Step-by-step explanation:
By definition a square is a four sided polygon (shape) which has four right angles and four sides of equal length.
So, when a rectangle has all congruent sides, its a square because its a rectangle so it has four right angles, and congruent sides are sides of the same length.
B is incorrect because a rectangle can have four right angles but have differing side lengths
C is incorrect because again a rectangle can have parallel sides but have differing side lengths
D is incorrect because it can have convex angles but again have differing side lengths
