The option that best describes the type of triangles necessary for justifying the SSS (Side-Side-Side) Congruence Theorem is:
Option #2: a pair of triangles with three pairs of congruent sides.
This is because the SSS Congruence Theorem states that if all three sides of one triangle are congruent to the three sides of another triangle, then the triangles are congruent. Thus, starting with triangles that have three pairs of congruent sides is essential for this theorem's justification.