Half of each circle is shaded, and the shaded half should turn steadily clockwise from one circle to the next. Which circle breaks the series?

Starting from (a), with the top half shaded, each circle should be the one before turned $45^{\circ }$ clockwise: top, top-right, right, bottom-right, bottom. Circle (b) has the top-LEFT half shaded, a turn the wrong way, so (b) is wrong.