Answer:
The correct option is a.
Step-by-step explanation:
Given: ∠1 and ∠2 are supplementary angles.
Prove: l║m
Proof:
It is given that ∠1 and ∠2 are supplementary angles.
.... (1)
∠1 and ∠3 are supplementary angles because both angles lie on a straight line.
.... (2)
Using (1) and (2), we get

Subtract angle 1 from both the sides.


Since ∠2 ≅ ∠3 are corresponding angles,

Hence proved.
Therefore the missing statement is
and option a is correct.