The orange cell slides down the diagonal from the top-left corner and the dot climbs the middle column. When either reaches the edge, it starts again from the other end. Which grid comes next?

The orange cell goes top-left, centre, bottom-right and then back to top-left, so next it is in the centre. The dot goes bottom, middle, top and back to the bottom, so next it is in the middle cell too, on top of the orange cell: (c). (a) leaves the dot at the top, (b) puts the cell in the wrong place and (d) moves the dot two cells.