"which answer is not a theorem or postulate used to prove triangles' similarity? "

There are 3 ways to prove that triangles are similar. Just pick the choice that is not listed.

1) AA - If two angles are congruent to the other 2 angles in a triangle, then the triangles must be similar.

2) SSS - If all the sides are in the same proportion, then the triangles must be similar.

3) SAS - If an angle between two proportional side is congruent to the angle in the other triangle, then the triangles must be similar.