Look at both the shape in the middle and the dot. Which figure comes next in the series?

The middle shape repeats in a cycle of 3 — circle, triangle, square — while the dot moves one corner clockwise each step (a cycle of 4). After circle, triangle comes square, and the dot moves from top-left to top-right: (a). (b) restarts the shape cycle too early, (c) forgets to move the dot and (d) repeats the triangle.