Set C is progressive alteration: the first 3x3 set, where one or more attributes follow an ordered series through the rows, the columns or both. Count, size, position, orientation, shape or fill steps by a fixed amount; sometimes it alternates or cycles; sometimes a row series and a column series cross.
What you see: something stepping across the grid. What you do: find the step, then find what the missing cell must be one step on from its neighbours.