Answer:
C. if ... congruent, then ... parallelogram
Explanation:
You want the converse of the statement ...
If a quadrilateral is a parallelogram, then the opposite sides of the quadrilateral are congruent.
Converse
The converse of the conditional if P, then Q is if Q, then P.
Here, we have ...
P = "a quadrilateral is a parallelogram"
Q = "the opposite sides of the quadrilateral are congruent"
So, the converse is ...
if Q, then P.
if the opposite sides of a quadrilateral are congruent, then the quadrilateral is a parallelogram. . . . . (matches C)