On reaction with dilute HCl, which of the following pair of metals will evolve hydrogen gas?

The NCERT chapter (Section 3.2.3, Activity 3.11) states that metals react with dilute acids as: $$\text{Metal} + \text{Dilute acid} \rightarrow \text{Salt} + \text{Hydrogen}$$ and gives the reactivity order towards dilute HCl as $Mg > Al > Zn > Fe$, adding that "In the case of copper, no bubbles were seen ... This shows that copper does not react with dilute HCl." The reactivity series (Table 3.2) places metals above hydrogen (K, Na, Ca, Mg, Al, Zn, Fe, Pb) as able to displace $H_2$ from dilute acids, while metals below hydrogen (Cu, Hg, Ag, Au) cannot.

Both magnesium and aluminium lie above hydrogen in the activity series, so both react with dilute HCl to evolve $H_2$ gas:

$$Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$$

$$2Al(s) + 6HCl(aq) \rightarrow 2AlCl_3(aq) + 3H_2(g)$$

Hence option D is correct — this is the only pair where BOTH metals are above hydrogen.

Why the other options are wrong:

• A. copper and zinc: Copper is below hydrogen in the activity series and, per the chapter, "does not react with dilute HCl"; only zinc would evolve $H_2$, not the pair.
• B. copper and iron: Again copper does not react with dilute HCl (it is below hydrogen); only iron reacts.
• C. silver and magnesium: Silver is one of the least reactive metals (below hydrogen, near gold at the bottom of Table 3.2) and does not react with dilute HCl; only magnesium reacts.