Explanation:

- Mercury and lead are hazardous chemicals that pose significant health risks to humans and ecosystems.

- Option 1: Noise Pollution

- Noise pollution involves excessive noise from sources like traffic and industrial activities.

- It doesn't involve chemical substances like mercury and lead.

- Option 2: Air Pollution

- While mercury and lead can become airborne, they primarily pollute the air as part of particulates, not as the main pollution form.

- Option 3: Water Pollution

- Mercury and lead can contaminate water bodies.

- They can enter waterways through industrial discharges and runoff.

- Option 4: Land contamination

- Mercury and lead often settle into soil due to improper disposal or industrial activities.

- They persist in the ground and harm plants and animals.

- Correct Answer: Option 4 – Land contamination