The square pattern is the same on both sides of its upright centre line and of its flat centre line. Which answer figure fills the missing quarter?

The top-right quarter is the mirror image of the top-left one in the upright line, and the bottom-left quarter is its mirror image in the flat line. So the bottom-right quarter is the bottom-left quarter mirrored left-to-right (or the top-right quarter mirrored top-to-bottom): the orange triangle sits in the bottom-right corner, the blue square near the centre and the green circle just below the middle line: (b). (a) and (c) copy a quarter without flipping it, and (d) turns the top-right quarter instead of flipping it.