Which of the following pair of metals react with very dilute HNO3 to evolve H2gas.(a) Pb, Cu(b) Na, K(c) Mg, Mn(d) Al, Zn

The metals that react with very dilute HNO3 to evolve H2 gas are Magnesium (Mg) and Manganese (Mn). So, the correct answer is (c) Mg, Mn.

Here's the step by step explanation:

1. Nitric acid (HNO3) is a strong oxidizing agent. It oxidizes the hydrogen gas produced during the reaction to water and itself gets reduced to any of nitrogen oxides (NO, NO2, N2O etc.). Therefore, metals usually do not release hydrogen gas upon reaction with nitric acid.

2. However, magnesium (Mg) and manganese (Mn) are exceptions to this behavior. They react with very dilute nitric acid and evolve hydrogen gas.

3. The reactions are as follows:

   Mg + 2HNO3 → Mg(NO3)2 + H2↑

   Mn + 2HNO3 → Mn(NO3)2 + H2↑

So, the correct answer is (c) Mg, Mn.