Which one of the following compounds has the highest boiling point?

Among isomeric amines, the boiling point decreases with an increase in the branching of the alkyl group.

Branching decreases the surface area of the molecule, which in turn decreases the magnitude of van der Waals forces of attraction.

n-Butylamine is a straight-chain primary amine and has the maximum surface area among the given isomers. Therefore, it has the strongest van der Waals forces and the highest boiling point.

Isobutylamine, sec-butylamine, and tert-butylamine have branched alkyl groups, leading to lower surface areas and consequently lower boiling points.

The order of boiling points is n-butylamine $>$ isobutylamine $>$ sec-butylamine $>$ tert-butylamine.

Answer: n-Butylamine