Question

which could be the missing reason in step 3?

Explanation:

Step1: Analyze the angles' positions

$\angle1$ and $\angle5$; $\angle3$ and $\angle6$ are alternate - interior angles.

Step2: Recall the alternate - interior angles theorem

When two parallel lines ($y\parallel z$) are cut by a transversal (line $AC$ for $\angle1$ and $\angle5$, line $AB$ for $\angle3$ and $\angle6$), alternate - interior angles are congruent.

Answer:

Alternate - interior angles theorem.