Why do atoms share electrons in covalent bonds? ?

Answer and Explanation:

Covalent bond is formed when electrons are shared by the atoms to fulfill their outermost shell.

In colvalent bond atoms shares electron to gain stability because if their outermost shell is not fulfilled then they are unstable.

So to gain stability electrons are shared by atoms in covalent bond , when they share electron their outermost shells are fulfilled and they become stable  

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.