The correct option is Magnesium.
Explanation
Generally, metals react with acids to displace hydrogen and evolve hydrogen gas. However, nitric acid ($HNO_3$) behaves differently because it is a strong oxidizing agent. In most cases, the hydrogen produced is immediately oxidized to water ($H_2O$), and the nitric acid itself is reduced to various nitrogen oxides (such as $N_2O$, $NO$, or $NO_2$).
Detailed Analysis:
- Magnesium ($Mg$): Magnesium and Manganese ($Mn$) are exceptions to the general rule regarding nitric acid. They react with very dilute nitric acid (approximately 1% concentration) to evolve hydrogen gas ($H_2$). The oxidizing power of very dilute nitric acid is insufficient to oxidize the hydrogen produced in this specific reaction.
- Copper ($Cu$): Copper does not evolve hydrogen gas with nitric acid. It reacts with dilute nitric acid to form copper nitrate, nitric oxide ($NO$), and water.
- Silver ($Ag$) and Gold ($Au$): These are noble metals located below hydrogen in the reactivity series. They do not displace hydrogen from acids. Gold does not react with nitric acid alone; it requires Aqua Regia (a mixture of nitric and hydrochloric acid) to dissolve.
Key Takeaway:
While nitric acid usually oxidizes hydrogen to water, Magnesium and Manganese are the specific exceptions that evolve hydrogen gas when reacting with very dilute nitric acid.