The answer is: a. The number of sodium ions is equal to the number of formula units of salt.
Balanced chemical equation: Na⁺ + Cl⁻ → NaCl.
Formula unit is the representative particle of an ionic compound. It is lowest whole number ratio of ions in an ionic compound, for sodium chloride it is NaCl.
In this chemical equation one sodium cation and one chlorine anion form one sodium chloride formula unit.