To find the point of symmetry:
⇒ must find where the distance between the original point and the
line of symmetry and reflected point and the line of symmetry are
equally distant from each other
Based on that knowledge:
⇒ the image appears to be entirely reflected horizontally
⇒ when the line of symmetry is the y-axis
⇒ A, A' equal distant from line of symmetry
⇒ B, B' equal distant from line of symmetry
⇒ C, C' equal distant from line of symmetry
Answer: y- axis
Hope that helps!