Orange cells fill the board in growing square blocks from the top-left corner. Which board comes next?

The blocks are 1 by 1, 2 by 2 and 3 by 3, holding 1, 4 and 9 cells. The next block is 4 by 4, which is the whole board, 16 cells: (a). (b) has only 12 cells, (c) has 15 and (d) is a staircase of 10 cells.