Question

which diagram shows $\triangle rst$ rotated $120^\circ$ counterclockwise about $s$?

Explanation:

Step1: Check rotation center

Rotation about S means S stays fixed. The top diagram has S' (moved), so it's invalid. The bottom diagram keeps S fixed.

Step2: Verify rotation direction/magnitude

Counterclockwise 120° from ΔRST about S should position R' and T' such that ∠R SR' and ∠T ST' are ~120°. The bottom diagram matches this.

Answer:

The bottom diagram