Estelle drew two parallel lines PQ and R0S intersected by a transversal KL, as shown below: Which theorem could Estelle use to show the measure of angle KMQ is equal to the measure of angle RNL?

Angle KMQ and angle RNL are on apposite sides of the transversal and between above and below (exterior to) the parallel lines, so they are alternate exterior angles. Theorem - Alternate exterior angles formed by parallel lines and a transversal have the same measure.