Structure of nitroalkanes,

Structure of alkyl nitrites,

Clearly, nitroalkanes are polar in nature and they have stronger dipole-dipole interaction than the corresponding alkyl nitrites.

We know that, Greater the dipole moment, greater will be the boiling point.

So, The boiling point of nitro-alkanes is higher than their corresponding alkyl nitrites.

Thus, both assertion and reason are true and reason is the correct explanation of assertion

Hence, option (A) is correct.