Which of the following is the reason for line 2 in the proof

reflexive is the answer.

Step-by-step explanation:

A relation is reflexive (has the reflexive property) if it applies to itself.

Example: The relation ≤ on numbers is reflexive because for any number x we have x ≤ x.  Thus any relation is said to be reflexive if for any a, (a,a) satisfies this relation.

Here we have to prove congruence of two triangles.  We find that the two triangles taken for congruence has the base RS as common.

Thus RS being the side for both the triangles is  making the statement two sides of the triangle are equal .  This is due to reflexive property.

Hence the two triangles are congruent by SSS