Detailed Explanation:

Answer: Option 4 — Aquatic plants absorb heavy metals and excess nutrients.

The analogy of wetlands as "kidneys of the Earth" is best reflected by their filtration function. Just as kidneys filter waste products and toxins from blood, wetlands act as natural filters by absorbing pollutants, heavy metals, and excess nutrients from water through aquatic plants and microbes. This phytoremediation and nutrient regulation process purifies water and prevents harmful substances from contaminating downstream ecosystems, paralleling the kidney's role in maintaining bodily health.

Why other options are less appropriate:
Option 1 describes the water cycle, which relates more to circulation than filtration. Option 2 explains the food web, not the purification function. Option 3 addresses sediment control, which is important but doesn't capture the filtering/cleansing aspect central to the kidney analogy.

📝 Short Notes: Wetland Functions

• Filtration & Purification: Wetlands remove pollutants, heavy metals, sediments, and excess nutrients through plant uptake and microbial action (phytoremediation and bioremediation).
• Nutrient Regulation: Absorb excess nitrogen and phosphorus, preventing eutrophication in water bodies downstream.
• Flood Control: Act as natural sponges, absorbing excess rainfall and reducing flood peaks.
• Groundwater Recharge: Facilitate percolation of water into aquifers, maintaining groundwater levels.
• Biodiversity Hotspots: Support diverse flora and fauna, including migratory birds, fish, and amphibians.
• Carbon Sequestration: Wetland vegetation and soils store significant amounts of carbon, helping mitigate climate change.
• Coastal Protection: Mangroves and coastal wetlands buffer against storms, tsunamis, and erosion.