Answer: B — Calcium
Calcium reacts with cold water and the hydrogen gas bubbles formed stick to its surface, causing it to float.
Source: Chapter 3, Section 3.2.2
The key detail from the passage is: "Calcium starts floating because the bubbles of hydrogen gas formed stick to the surface of the metal." Sodium and potassium also react with water but they react violently (hydrogen catches fire) — they don't simply float. Iron does not react with cold or hot water at all. Calcium is the correct answer because it reacts moderately and floats due to hydrogen bubbles.