Reactivity of an element is its readiness to lose or gain electrons. Fluorine is the most reactive halogen, it readily combines with metals. Rank the reactivity of fluorine with lithium (Li), rubidium (Rb), sodium (Na) and potassium (K) from lowest to highest reactivity. Explain the ranking based on your understanding of periodic trends

what is the question here. i think you messed up.

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.