Answer:
B. When its angles are right angles.
Explanation:
A rhombus is a quadrilateral with all 4 sides congruent. The sides can be slanted, which would keep it from being a square. If you make all the sides of a rhombus straight vertically and horizontally so you have 4 right angles, you have a square. A square is a quadrilateral with all 4 sides AND all 4 angles congruent.