All sides are congruent, opposite sides are parallel, and opposite angles are congruent (but not 90) describes which shape

(because 360-180=180). Opposite angles of parallelograms are congruent, which means that each other the missing angles must have a measure of 90° (since 180:2=90). This tells us that there are actually four right angles in Parallelogram C, so we know that it is a square (and a rhombus).