(D) Magnesium
Magnesium does not react with cold water. It reacts only with hot water to form magnesium hydroxide and hydrogen gas.
Source: Chapter 3, Section 3.2.2
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The textbook clearly states: "Magnesium does not react with cold water. It reacts with hot water…" Sodium, potassium, and calcium all react with cold water (K and Na vigorously; Ca less violently). This is a common MCQ testing knowledge of the reactivity series — remember the order K > Na > Ca > Mg for reactions with water.