△ABC is reflected to form​​ ​ △A′B′C′ ​. The vertices of △ABC are A(3, 1) , B(1, 5) , and C(6, 9) . The vertices of △A′B′C′ are A′(−1, −3) , B′(−5, −1) , and C′(−9, −6) . Which reflection results in the transformation of ​ △ABC ​​ to ​ △A′B′C′ ​​? reflection across the x-axis reflection across the y-axis reflection across y = x reflection across y=−x

Reflection refers to a rigid motion or transformation which occur and creates a mirror image to the given figure. 

Reflections can be across the x-axis meaning that the result of the image is going to be (x,y) to (x,-y), and across the y-axis where the preimage (x,y) would be reflect to result in the image of (-x,y).
Another to kinds of reflection are across the line y=x where preimage (x,y) results in the new point (y,x) and the reflection across the line y=-x where the transformation results in the preimage (-y,-x).

Is given that the triangle ABC was reflected to create the new triangle A'B'C'. The coordinates of the vertices of the triangle ABC are A(3,1), B(1,5), and C(6,9), the new vertices after the reflection of the preimage are A'(-1,-3), B'(-5,-1), and C'(-9,-6).

After the previous said, the transformation occur was a reflection across the line y=-x. You can check this answer by selecting a point, and then looking at the rule for this kind of reflection.