When an element 'X' reacts with water, it starts floating. Identify the element 'X':

According to the chapter's discussion of Activity 3.10: "The reaction of calcium with water is less violent. The heat evolved is not sufficient for the hydrogen to catch fire... Calcium starts floating because the bubbles of hydrogen gas formed stick to the surface of the metal." The reaction is:

$$Ca(s) + 2H_2O(l) \rightarrow Ca(OH)_2(aq) + H_2(g)$$

• (B) Calcium — Correct. It reacts with cold water, forms bubbles of $\mathrm{H_2}$ gas that stick to its surface, making it float — exactly matching the description in the question.
• (A) Potassium — Incorrect. Potassium reacts violently and exothermically with cold water; the evolved hydrogen immediately catches fire ($2K + 2H_2O \rightarrow 2KOH + H_2 + \text{heat}$), so it does not simply float — it is dangerously reactive.
• (C) Sodium — Incorrect. Like potassium, sodium reacts violently with cold water, the reaction being exothermic enough for the hydrogen produced to catch fire; it does not merely float.
• (D) Iron — Incorrect. Iron does not react with cold or hot water at all; it reacts only with steam to form $\mathrm{Fe_3O_4}$ and hydrogen, so it would not float due to gas bubbles in water.

Hence calcium is the element X.