Density is defined as the comparative ratio of a substance's

Density is defined as the comparative ratio of a substance's mass per unit volume.

The formula of density of a substance is as follows:

Density=\frac{Mass}{Volume}

Let the unit of mass be grams and that of volume be mL thus, unit of density will be g/mL. For density value x g/mL, there are x grams of substance in 1 mL volume. Thus, density is the comparative ratio of mass per unit volume of a substance.

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.