A. angle addition postulate with angles forming a straight line angle
1) Examining that table, we can see that step 4 is a consequence of the third step, the triangle sum theorem.
2) Then in step 4, we have the following reason to state that the sum of those angles is 180º: Then as we can see below:
We have a Linear Pair between the angles ∠ABD, ∠DBE, and ∠CBE since those angles combined add up to 180º (a straight angle) in red.
3). Hence, the answer is A