(a) The water cycle in wetlands involves surface runoff, subsoil percolation and evaporation.

(b) Algae form the nutrient base upon which fish, crustaceans, molluscs, birds, reptiles and mammals thrive.

(c) Wetlands play a vital role in maintaining sedimentation balance and soil stabilisation.

(d) Aquatic plants absorb heavy metals and excess nutrients.

5. Ans: d

Explanation:

According to Ramsar convention wetlands are defined as “areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six metres”. 

Significance of Wetlands:

• Wetlands are amongst the Earth’s most productive ecosystems. These delicate environments have significant ecologic and economic values. 
• Wetlands ecosystems are vital parts of hydrological cycle, highly productive, support rich biodiversity and provide a wide range of ecosystem services such as water storage, water purification, flood mitigation, storm buffers, erosion control, aquifer recharge, microclimate regulation, aesthetic enhancement of landscapes while simultaneously supporting many significant recreational, social and cultural activities.
• They have been described both as “the kidneys of the landscape”, because of the functions they perform in the hydrological and chemical cycles, nutrient cycling, their high and long-term capacity to filter pollutants including heavy metals and trap sediments from the water that flows through them. 
• Wetlands store water to ensure supply during dry periods. Wetlands work like giant sponges. They store water and then slowly release it and this helps to deal with dry seasons with little rainfall.
• They are called “biological supermarkets” because of the extensive food webs and rich biodiversity they support.

Hence, option d is correct.

Read: Previous Year UPSC Environment Questions (PYQs) With Explanation 2022