Jazz draws a transversal, t, on two parallel lines ab and cd, as shown below:

(first pic)

he makes the following table to prove that the alternate interior angles are equal:

statement justification
angle 2 = angle 6 corresponding angles of parallel lines are congruent.

angle 2 = angle 4 ?

angle 4 = angle 6 transitive property of equality,
angle 2 = angle 6,
angle 2 = angle 4,
therefore angle 4 = angle 6

which is the missing justification?

angles 1 and 3 are supplementary; therefore, angle 2 is equal to angle 4.
alternate interior angles are congruent.
vertical angles are congruent.
angles 1 and 4 and angles 1 and 2 are congruent; therefore, angle 2 is equal to angle 4.