Answer:
Step: 3 
Reason:Alternate interior angles theorem.
Step-by-step explanation:
We are given that
and both lines are cut by transversal t.
We have to find third step in proof.
Proof:
Step 1:
Reason :Given
Step 2:
Reason: Verticale angles theorem.
Step: 3 
Reason:Alternate interior angles theorem.
Step 4:
Reason: Transitive property of congruence
Step 5:
Reason:Vertical angles theorem
Step 6:
Reason:Transitive property of congruence.
Hence, proved