Explanation:

Let’s look at each option:

- Cu²⁺: Zinc is above Copper in the electrochemical series, so Zn can reduce Cu²⁺ to Cu metal.

- H⁺: Zinc can reduce H⁺ ions to H₂ gas (seen in reactions with acids).

- Ag⁺: Zinc is above Silver, so Zn can reduce Ag⁺ to metallic Ag.

- Al³⁺: Zinc cannot reduce Al³⁺. Aluminum is above zinc in the electrochemical series, so Zn cannot reduce Al³⁺ to Al.

So, the correct answer is:

Option 4 – Al³⁺