To determine the measure of angle C there are two approaches, one is using the angles that are over the same line (red) as it, and considering that they are suppplementary, i.e. they add up to 180º, calculate the value of C as:
The other aproach is by identifying its vertical oposite angle, that is, an angle that shares vertex with C and is in the oposite side of the X the lines form when they cross (green).
Vertical oposite angles are congruent, so
∠C=90º
Using either way you reach the same result.
So the correct option is the third one.