The answer is Definition of a segment bisector; SSS.
Step-by-step explanation:
Since RT bisects QS, we have QT congruent to ST.
By SSS, we have triangle QRT congruent to triangle SRT
so, since QT is congruent to ST, RT is congruent to RT, and QR is congruent to SR.
All these sides are congruent to each other that gives the proof of "SSS(SIDE, SIDE, SIDE)"