An unknown substance 'S' reacts vigorously with cold water, air and hydrochloric acid. It remains unaffected by sodium hydroxide base. Identify the substance amongst the following options.

The correct option is D Sodium

• Sodium is a very reactive metal. It reacts vigorously with cold water, air and acids. The reaction with water and acids is violent and the hydrogen which is evolved as a product may immediately catch fire. Sodium does not react with sodium hydroxide base.
• Metals like aluminium and iron, which are moderately reactive do not react with cold water. They react with steam to form the metal oxide and hydrogen.
• Phosphorous is a non-metal. It does not react with water or acid.