When a zinc rod is kept in a copper nitrate solution, a redox reaction takes place between the zinc metal and the copper ions in the solution. This reaction is known as a displacement reaction. Let's understand the process in detail:

1. Formation of Copper Nitrate Solution:
- Copper nitrate (Cu(NO3)2) is a soluble compound that dissociates into copper ions (Cu2+) and nitrate ions (NO3-) when dissolved in water.
- The solution contains copper ions that are free to move and react with other substances.

2. Redox Reaction between Zinc and Copper Ions:
- When a zinc rod is immersed in the copper nitrate solution, the zinc atoms on the surface of the rod lose electrons and are oxidized to form zinc ions (Zn2+).
- The copper ions in the solution gain these electrons and are reduced to form copper metal.
- This reaction can be represented by the following equation:
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)

3. Formation of Copper Deposit:
- As the redox reaction proceeds, copper ions from the solution are reduced and deposited onto the surface of the zinc rod.
- This deposition of copper metal onto the zinc rod is known as electroplating or electrodeposition.
- The copper metal forms a layer on the surface of the zinc rod, and this layer gradually thickens as more copper ions are reduced and deposited.

4. Explanation of the Correct Answer:
- Option B, "copper is deposited on zinc," is the correct answer because during the redox reaction, copper ions in the solution are reduced and deposited onto the surface of the zinc rod.
- The zinc rod acts as the reducing agent, losing electrons and undergoing oxidation, while the copper ions act as the oxidizing agent, gaining electrons and undergoing reduction.
- As a result, copper metal is deposited on the zinc rod, and the solution loses copper ions.

To summarize, when a zinc rod is kept in a copper nitrate solution, a redox reaction occurs. The zinc rod is oxidized, releasing electrons, while the copper ions in the solution are reduced, depositing copper metal onto the zinc rod. This process is commonly observed in electroplating applications.