A highly reactive element X is stored under water .It readily reacts with oxygen of air to give a compound Y which dissolves in water .The aqueous solution of Y changes blue litmus to red.The element X is a) Na b) S c) P d) K
It is phosphorous.
Sodium and potassium form bases which turns red litmus blue
Sulphur is not very reactive.
So, phosphorous reacts with oxygen to form phosphorous pentoxide which changes blue litmus red.