Question

segments are congruent if and only if they have the same measure
segment addition postulate
reflexive property of congruence
definition of congruence
symmetric property of congruence

Explanation:

The statement "Segments are congruent if and only if they have the same measure" is the definition of congruence for segments. The Segment Addition Postulate is about adding segments on a line. The Reflexive Property of Congruence states a segment is congruent to itself. The Symmetric Property of Congruence is about reversing congruence statements.

Answer:

Definition of Congruence