If angle C is 120
degrees, what is the
measure of angle D?

If angle C is supplementary to angle D, then angle D is 60 degrees.

If angle C is corresponding, vertical, alternate interior, or alternate exterior to angle D, then angle D is 120 degrees.

this may not be at all, but i use the same website you're using for school, and if i'm stuck doing one of these i try to look for it on the section.