When does a parallelogram become a rectangle?
When all four internal angles become right angles.
A parallelogram is a quadrilateral whose opposite sides are parallel and equal, and whose opposite angles are equal; however, its angles are generally oblique and need not measure 90°.
A rectangle is defined as a special parallelogram in which every interior angle is a right angle, so the condition that converts one into the other lies in the angles, not the sides.
In any parallelogram the co-interior (adjacent) angles are supplementary, adding up to 180°.
Therefore, the moment a single interior angle becomes 90°, its neighbour must also be 90°, and by the opposite-angle property all four angles become right angles automatically.
This makes the sides meet perpendicularly and the figure satisfies the rectangle definition exactly.
Making two adjacent sides equal instead produces a rhombus, and perpendicular diagonals likewise describe a rhombus or square, so those options do not define a rectangle.
Unequal boundary sides do not follow the parallelogram rule at all, since opposite sides must stay equal.
Hence, the answer is When all four internal angles become right angles.
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