The orange cell moves round the edge of the grid and every cell it leaves turns blue. Which grid comes next?

Each step the orange cell moves one cell clockwise along the edge and the cell it just left is coloured blue, so the blue trail grows by one. Next the orange cell is in the bottom-right corner with four blue cells behind it: (b). (a) forgets to colour the trail blue, (c) leaves a gap in the trail and (d) moves the orange cell two places.