Every small grid has one orange cell and one blue circle. Follow them along the rows and down the columns. Which small grid is missing?

Moving right along a row, the orange cell steps one cell right and the blue circle one cell left; moving down a column, the orange cell steps one cell down and the circle one cell up, always wrapping round at the edges. So the last grid has the orange cell in the bottom-right corner and the circle in the bottom-left corner: (d). (b) swaps the two, (c) leaves the circle where it was and (a) puts the circle in the top row.