Answer:
The correct option is 2. The triangles are congruent because △R′S′T′ is a rotation of △RST, and a rotation is a rigid motion.
Explanation:
It is given that △RST maps to △R′S′T′ by using the rule

Since the absolute values of x and y-coordinates are same but the sign of both coordinates are changed. It is possible, if the figure is rotated 180 degree about the origin.
When a figure rotated 180 degree about the origin, then

Rotation is a rigid motion and image and preimage are congruent to each other.
Therefore option 2 is correct.