Dots are added to the corners of the square one at a time, going round in a fixed order. Which figure is next?

One dot is added per step, going clockwise round the corners: top-left, top-right, bottom-right, bottom-left. After every corner has one dot, the round starts again at the top-left, which now gets a second dot: (a). (b) starts the new round at the wrong corner, (c) adds four dots and (d) goes anticlockwise.