The correct option is D Functional group isomerism
For a compound with molecular formula CcHhXxNnOo
Degree of unsaturation=(c+1)−h2−x2+n2
Where, X = Halogen
c=number of carbon atom
h=number of hydrogen atom
n=number of nitrogen atom
o=number of oxygen atom
In C3H6O
Degree of unsaturation=(3+1)−62=1
Degree of unsaturation=1
No possible metamerism and positional isomers are possible.
Possible function group isomers are
CH3–CH2–CH=O & CH3–CO–CH3. Therefore, functional group isomerism is the most appropriate.