The wheels are shaded clockwise starting just right of the top. Study each row of wheels and choose the wheel that completes the grid.

In every row the number of shaded parts in the third wheel is the sum of the first two: $1+1=2,1+2=3$. The bottom row needs $2+1=3$ parts shaded, starting just right of the top and going clockwise: (b). (a) and (c) have 2 and 4 parts, and (d) has 3 parts but starts in the wrong place.