ANS:B - Sodium

When sodium comes into contact with water, a highly exothermic reaction occurs, leading to the rapid production of hydrogen gas and the formation of sodium hydroxide. The reaction is so vigorous that the released heat ignites the hydrogen gas, resulting in a flame. Additionally, the heat generated by the reaction may cause the sodium metal to melt and move around rapidly on the water's surface, creating a sizzling or popping sound. The reaction of sodium with water can be represented by the following chemical equation: 2Na(s)+2H2​O(l)→2NaOH(aq)+H2​(g) This reaction is highly exothermic and violent, often resulting in the rapid evolution of hydrogen gas and the dispersion of small particles of sodium metal. Due to its extreme reactivity with water, sodium is typically stored under oil to prevent contact with moisture in the air. In contrast, while calcium and magnesium also react with water, their reactions are not as violent as that of sodium. Mercury is a liquid at room temperature and does not react with water under normal conditions. Therefore, among the options provided, sodium is the metal that reacts violently with water.