Which of the following pairs of elements is most likely to form an ionic compound?
a. sodium and bromine
b. nitrogen and phosphorus
c. oxygen and fluorine
d. lithium and helium

Option A = sodium and bromine

Explanation:

Sodium is metal while bromine is non metal. The bond formed between them is ionic.

Ionic bond:

Ionic bond is formed by the complete transfer of electron form metal to non metal.

Sodium is belongs to the group one while bromine is present in group seventeen. The number of valance electron of bromine are seven. Bromine require one electron to complete the octet, while sodium lose its one valance electron to gain noble gas stable electronic configuration. Thus sodium transfer its electron to bromine and gain positive charge and bromine gain its electron and form negative ion. The forces present between them are electrostatic forces which are very strong.

Na⁺Br⁻

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.