Solution:concept - Sodium is a metal with the symbol Na and its atomic number is 11. It is a soft, silvery-white, highly reactive metal.
Sodium is an alkali metal that belongs to group 1 of the periodic table. A free metal does not occur in nature and must be prepared from compounds.
Thus, the most common form in which sodium exists is its halide ore. Rock salt or common salt is the halide ore of sodium. It is majorly obtained from seawater. The chemical formula of halide is .
Hence, the correct answer is 3.