(C) Hydrogen
Magnesium (Mg) reacts with very dilute HNO₃ to evolve H₂ gas, unlike other metals where HNO₃ (a strong oxidising agent) oxidises H₂ to water.
The textbook explicitly states: "But magnesium (Mg) and manganese (Mn) react with very dilute HNO₃ to evolve H₂ gas." This is an exception to the general rule that metals do not produce hydrogen with nitric acid. Examiners expect students to recall this specific exception.