The correct option is C I, III and IV
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 C5H10O
Degree of unsaturation=(5+1)−102=1
Compound has one degree of unsaturation, it can be an aldehyde, ketone, unsaturated alcohol, etc., but can't be an ester as it contains two oxygen atoms.
2-methylbutanal and Pentanal are chain isomers of each other.
Aldehyde and ketone are functional isomers. Hence, option c is correct.