Which set of atoms will form an ionic compound?

Explanation:

An ionic compound is formed when one atom donates one or more electrons to one or more atoms of another element. Calcium has two valence electrons, which are donated to an atom of oxygen. So, an ionic compound is formed in the reaction involving one calcium atom with an oxygen atom.

a is the correct answer

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.