In an ionic substance, mole ratio of cation to anion is 1 : 2. If atomic mass of metal cation and non-metal anion respectively are 138 and 19 amu then, the most correct statement preferentially is

Mass of Metal cation (A) is 138 amu

Mass of Non-metal anion (B) is 19 amu

The mole ratio of cation to anion is 1:2 implies the formula of ionic substance is $A{B}_2$

The formula mass of the substance $=138+(19\times 2)=176 amu$ 

Ionic substances should not be called as ‘compounds’ as they do not contain discrete molecules. Mass of such a substance has to be referred as ‘Formula Mass’ and not  as ‘Molecular Mass’