A staircase of yellow blocks is built step by step. Which figure is the next stage?

Each stage adds a new column on the right that is one block taller than the last: columns 1; 1, 2; 1, 2, 3; 1, 2, 3, 4. The next stage has columns 1, 2, 3, 4, 5 rising to the right, i.e. $1+2+3+4+5=15$ blocks: (d). (b) climbs the wrong way and (a) adds a block on top of every column instead.