Answer:
The missing step is A.
Step-by-step explanation:
There are several properties about parallel straight lines that you need to bear in mind:
The angles are congruent, in other words they are the same. Angles on top of the lines are equal. Similarly, angles beneath the lines are equal.
The angles form Z angles with the transverse line. These angles are equal as well.
These properties make parallel lines to produce equal angles. Hence the proof.