Answer:
 C.  Vertical angles are supplementary if both are right angles.
Step-by-step explanation:
You want to know the conditions when vertical angles are supplementary.
Two angles are supplementary when they total 180°.
Vertical angles are formed when two lines cross. They share a vertex, but not a side. Their sides are opposite rays.
Vertical angles are always congruent.
If x is the measure of one of two supplementary vertical angles, then we must have ...
 x + x = 180°
 2x = 180° . . . . . collect terms
 x = 90° . . . . . . . divide by 2
Vertical angles will be supplementary if and only if they are both right angles.