Of two similar triangles, the area of one is 9 times the area of the other.
By what percentage is the perimeter of the larger triangle greater than the perimeter of the smaller?

An area factor of 9 gives a length factor of $\sqrt{9}=3$, and a perimeter is a length, so the larger perimeter is three times the smaller. Three times means an increase of 200 per cent, not 300 per cent — an increase of 300 per cent would make it four times as large. The option 800 per cent is the increase in AREA, and 900 per cent quotes the new area as a percentage of the old.