Which of the following statements would be the reason in line 4 of the proof? a.) definition of supplementary b.) two < 's supplementary to equal < 's are = c.) substitution

Option B.

Step-by-step explanation:

∠1 and ∠3 are supplementary and ∠2 and ∠4 are supplementary.

Because they are exterior sides in opposite rays.

In other words ∠1 + ∠3 = 180° and ∠2 + ∠4 = 180°

and it is given that ∠1 ≅ ∠2

So ∠3 ≅ ∠4

Since Two angles supplementary to equal angles are equal will be the reason.

Option B is the correct option.