Each circle is fully shaded, half shaded or empty. The half-shaded circles are not all the same either. Choose the circle for the empty cell.

Every row and every column has one full, one half-shaded and one empty circle, so the bottom-right cell needs a half-shaded circle. The shaded half turns a quarter turn anticlockwise from row to row: right half in the top row, top half in the middle row, so the left half in the bottom row: (b). (a) shades the right half, and (c) and (d) repeat a full or empty circle.