Propylamine and trimethylamine have the same molecular formula, c 3 h 9 n, but different structures.
which of these substances is expected to have the higher boiling point? why?

The structure of propylamine is pretty linear CH3-CH2 - CH2 - NH2.

The structur of trimethylamine is completely branched: a atom of N surroundeb by three CH3- radicals.

As theory predicts, in absence of other factors, longer chains have higher intermolecular forces  (London forces) than branched compounds. So we must expect that propylamine has higher boiling point methyleamine.

You can verify that prediction with the literature. I did it: propylamine boiling point is 47-51°C while methylamine boiling point is 3 - 7°C.

Ibelieve it is b. proton.hope my answer has you : )