Half of each circle is shaded. Which circle should come next in the series?

The dividing line turns $45^{\circ }$ anticlockwise each time, so the shaded half moves from the right towards the top, and the colour switches orange, blue, orange, blue. Next the shaded half is on the left and orange: (d). (a) forgets to switch the colour; (b) puts the shading on the right.