The correct order of the mobility of the alkali metal ions in aqueous solution is

A solid compound X on heating gives CO₂ gas and a residue. The residue mixed with water forms Y. On passing an excess of CO₂ through Y in water, a clear solution Z is obtained. On boiling Z, compound X is reformed. The compound X is

In which of the following processes, fused sodium hydroxide is electrolysed at a 330^oC temperature for extraction of sodium?