Dots are stacked into triangles that grow from box to box. Which answer box shows the next triangle?

The triangles hold 1, 3, 6 and 10 dots: each new row has one dot more than the row above $(+2,+3,+4)$. The next triangle adds a row of 5 dots, giving $10+5=15$ dots: (c). (a) has only 14 dots, (b) is a square of 16 dots and (d) adds two rows (21 dots).