All rhombuses are parallelograms, but not all parallelograms are rhombuses. what attribute does a rhombus have that a parallelogram does not? a) 4 congruent sides b) 2 sets of opposite parallel sides c) 2 sets of opposite congruent angles d) 4 angles that have a sum of 360°

The answer is:  [C]:  "2 sets of opposite congruent angles" .

{Note: 
All squares are rhombuses; but not all rhombuses are squares.
All squares are parallelograms, but not all parallelograms are squares.
All squares are rectangles, but not all rectangles are squares.                                       All rectangles are parallelograms, but not all parallelograms are rectangles.}.