pattern - Complete the sequence

As the title says it is a compete the sequence.

My thoughts:

From all the circle ones that aren't a circle, you can make a circle by rotating the pieces. From all the triangle ones that aren't a triangle, you can make a triangle by moving one line. But I can't figure out a rule for the square-ish things. (Of course the rules I came up with for the circle and triangle might be wrong.)

Answer

I'll go with:

Why?

There's a triangle, a circle and a 'U' shape. They each appear both 'normal', once with both halves mirrored along a vertical axis, and once with the left side intact and the right rotated 180 degrees.

Based on what's there already:

the three states for triangle and circle, and both the normal and mirrored state of the 'U' shape

what we need is:

left side of the U intact, right side rotated 180 degrees.