Jarvis left out a statement from the proof shown.which statement is best explained by the corresponding angles postulate

The postulate of the corresponding angles establishes that when a transversal line cuts two parallel lines, the corresponding angles are congruent. These angles are on the same side of the parallel lines and on the same side of the transversal line.

  Then, if we based on this definition and analize the figure attached, we can notice that the angles ∠1 and ∠3 are corresponding angles, so they are congruent. In this case the angle ∠1 is internal and the angle ∠3 is external.

 The answer is: ∠1 and ∠3 are congruent (See the image attached).