Answer:
B: ∠1 ≅ ∠2
Explanation:
Transitive property of congruence says that
If a=b and a=c then b=c
From the given we can see that ∠1 ≅ ∠2
Vertical angles are congruent. so ∠1 ≅ ∠3
By Transitive property of congruence
If ∠1 ≅ ∠2 and ∠1 ≅ ∠3 then ∠3 ≅ ∠2
The missing statement in the proof is the statement already given for the diagram that is
∠1 ≅ ∠2