Which among the following is a pair of functional isomers?

Which among the following compounds has highest boiling point?

Calculate the molality of solution of nonvolatile solute if boiling point elevation is 1.75 K .

[ $\mathrm{K}_{\mathrm{b}}$ for solvent $=3 \mathrm{~K} \mathrm{~kg} \mathrm{~mol}^{-1}$ ]

Which from following pair of compounds exhibits isomorphism?