Answer:
C. 123°
Step-by-step explanation:
First, we see that the red arrows indicate the two horizontal lines are parallel. Parallel lines have certain properties with regards to corresponding angles.
Notice that the angle labelled with a question mark "?" and the angle marked 123 degrees are same-side angles, or corresponding angles. By definition, then, they are equivalent in measure.
Thus, the indicated angle is also 123 degrees, so the answer is C.
