Hydrogen can be displaced from a hot alkaline solution by

Analyze the question: We need a metal that can displace hydrogen from a hot alkaline solution (e.g., hot NaOH). This is a property of amphoteric metals, which react with both acids and bases.

Concept: Amphoteric metals such as Zn, Al, Sn and Pb react with hot concentrated alkalis to give hydrogen gas and a salt (a zincate, aluminate, stannate or plumbate).

Examine each option:

• A. Fe: Iron is not amphoteric. It does not react with alkalis, so no hydrogen is released.
• B. Cu: Copper is below hydrogen in the reactivity series and does not react with alkalis.
• C. Ca: Calcium is very reactive and displaces hydrogen from cold water and acids. It forms Ca(OH)₂, a base, and does not behave amphoterically. The question is about a hot alkaline solution, where the key property is amphoterism, which calcium lacks.
• D. Sn: Tin is amphoteric. It reacts with hot concentrated alkali:

Sn + 2NaOH + 2H₂O → Na₂Sn(OH)₄ + H₂↑

(or, written another way, Sn + 2NaOH → Na₂SnO₂ + H₂)

Answer: The correct option is D (Sn).

Tip: Remember the amphoteric metals with the phrase "Zn, Al, Sn, Pb" (think "ZAPS"). Whenever a question mentions a metal reacting with an alkali such as NaOH, look for one of these.