Each square is divided into four quarters. Study how the shading grows along a row and how it moves down a column. Which square is missing?

Along a row one more quarter is shaded each time, going clockwise. Down a column the whole pattern turns a quarter turn clockwise. So the bottom row starts at the bottom-right quarter, adds bottom-left, and then adds top-left: (b). (a) adds the top-right quarter (anticlockwise), (c) adds nothing and (d) starts from the wrong quarter.