The coloured cells move together across the grid, and something else changes at each step too. A cell that leaves one side comes back on the other side. Which grid comes next?

At each step the whole group of cells moves one column to the left (a cell leaving the left edge comes back at the right edge), and the orange cells turn blue while the blue cell turns orange. Doing both once more gives (a). (b) moves the group right, (c) forgets to swap the colours and (d) turns the grid.