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

c is the answer because when something is left after something evaporates that shows the chemical change.