Astable, binary ionic compound exists where the cation (a) has a charge twice in magnitude than that of the anion (x).
what would be the correct formula for this compound?

AX₂

Explanation:

Binary compounds:

The compounds which are made up of the atoms of only two elements are called binary compounds.

For example:

The following compounds are binary:

HCl

H₂O

NH₃

HCl is binary because it is composed of only hydrogen and chlorine. Ammonia is also binary compound because it is made up of only two elements nitrogen and hydrogen.

water is also binary because it is also made up of only two elements hydrogen and oxygen.

In given question it is stated that the charge of cation is twice than the anion which means its charge would be +2 thus anionic part will have the charge -1. In neutral compound the positive and negative charges must be equal in magnitude to cancel the effect of each other and make the compound neutral. So, there must be two anionic atom present so that charge becomes -2 and cancel the +2 charge of cationic part. Thus the formula will be,

AX₂

A⁺²X₂⁻²

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.