During bromination using NBS, the addition product is not observed because the rate of addition is than the rate of the chain propagation steps.

slower

Explanation:

NBS is used in place of Br2 during bromination because the use of Br2 tends to result in the formation of a large concentration of the addition product.  This is because, the Br2 interacts with the double bond to yield only the dibromide (addition product).

The usefulness of NBS lies in the fact that it provides a lower concentration of Br2, and such that the rate addition is slower than the chain propagation steps. Only the monobromination product is observed

The low concentration of Br2 allows the free-radical reaction to out-compete the alkene addition reaction.

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.