When the vapor pressure of a liquid is equal to the atmospheric pressure, the liquid

The liquid will boil.

Explanation:

Boiling will occur when the vapor pressure is equal to the atmospheric pressure. This is called the boiling point. Without any external pressure the liquid molecules will be able to spread out and change from a liquid to a gas.

what is the question?

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.