how is this marked wrong wouldnt it be 18 cm ?

Answer:
14 cm
Explanation:
Oh, this is a trick question! The thing is, the plant was already 4 cm on Day 0. So from Day 0 - Day 15, the plant actually only grew (18 - 4 =) 14 cm.
They got you this time, but hopefully you know to watch out next time. I hope this helps!