Answer: one ring
Explanation:
C15H28O2: DU= (2(m)+2-n)/2
m is the no of Carbon atoms and n is no of Hs in the given Molecular Forumla.
DU=2(15)+2-28/2= 32-28/2=2
It mentioned that it has one double bond and it corresponds to one DU.
And remaining one DU corresponds to one ring