I think it is
the right one in the top row.
there are three basic shapes on the 9 images: the circle (top row middle), the triangle (second row middle), and the U-shaped thingy (second row right).
They all appear in three forms:
1. the basic variation
2. a version in which the basic shapes right and left sides are swapped
- two facing half circles
- the rotated K
- and the upside-down T
3. and a variation in which their right half is rotated 180 degrees
- the CC-looking stuff
- the bottom middle one
- and... the one that is missing, so it must be a half U followed by the otherhalf of the half U, rotated upside down
An answer that's at least plausible is
There are three groups of shapes, the circle thingies, the triangle thingies, and the squarish thingies.
To go from the first-row shape to the second-row shape, you
halve the picture vertically, then swap the halves.
To go from the first-row shape to the third-row shape, you
halve the picture vertically, flip the left half vertically, the right half horizontally, then flip the whole thing horizontally.