Read the description of the equation carefully: the flower's height h is equal to 12 + 2.25w, where w is the number of weeks that pass.
If w = 0, this means no time has passed, and so h would be the flower's starting height:
12 + 2.25•0 = 12 + 0 = 12
After 1 week, w = 1, its height would be
12 + 2.25•1 = 12 + 2.25 = 14.25
After another week, w = 2, its height would be
12 + 2.25•2 = 12 + 4.5 = 16.5
and so on.
For every week that passes, the flower grows another 2.25 inches, which is exactly what the last option is saying.