Answer
(b) Na$_{2}$SO$_{3}$
Work Step by Step
Sodium's elemental symbol is Na$_{+}$. The symbol for sulfite is SO$_{3}^{2-}$. Sodium has a +1 charge whereas sulfite has a -2 charge. For the compound between sodium and sulfite to have a neutral charge, the charges must equal. We need two positive charges to counter the -2 charge of sulfite. We then need to combine two sodium atoms for every sulfite ion.
The molecular formula for sodium sulfite is option (b) Na$_{2}$SO$_{3}$.
