Answer
a) Same molecule
b) Isomers
c) Same molecule
Work Step by Step
a) Here, both the molecules are same. Both have same structure and same molecular formula. Both the molecules have same name : 2,3-dimethylbutane.
b) These two molecules have different structure although their molecular formula is same (C_{4}H_{8)O). They have also different IUPAC name. One is Butanal and other is 2-methyl-1-propanal. So, these are isomers.
c) These two molecules have same structure and molecular formula. So, they are same molecule. Both have the same name: 3-methylpentane.
