If the reflection line is the Y axis, change (x, y) by (-x, y)
We have then:
R (6, 6) ---> R '(- 6, 6)
S (3, -6) ---> S '(- 3, -6)
T (0, 3) ----> T '(0, 3)
Answer:
The new vertices are
R '(- 6, 6)
S '(- 3, -6)
T '(0, 3)
Graph four (last from left to right) is the correct one.