Answer:
AA similarity postulate
Step-by-step explanation:
Triangles are similar if corresponding angles are congruent or if corresponding sides are proportional.
Here, the missing reason in the proof follows a step in which two angles are shown congruent. That means you can claim similarity by the AA similarity postulate.
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Additional comment
The similarity postulates include ...
AA -- two corresponding angles (the third angle is determined by these two)
SSS -- three proportional corresponding sides
AAS -- two congruent angles and a proportional corresponding side
ASA -- a variation of AAS
SAS -- proportional corresponding sides flanking a congruent angle
With the exception of AA, these are the same postulates as used to prove triangle congruence when the corresponding sides are congruent, rather than proportional.
