How the rule works
The third cell in every row is the first one mirrored: what sat on the left sits on the right, or what was on top is at the bottom. Everything flips at once, the lines along the edge, the symbols in a container, the dots on either side of a line.
A mirroring is not a rotation. A turned L and a mirrored L look different, and the options in such a task often differ in exactly that. Find something that breaks the symmetry, a single line or a dot, and see where it ends up.