Biodegradable pollutant is

The correct answer is B) Sewage. Sewage is organic waste from households and industry, made up of human excreta, kitchen and bathroom wastewater. Microorganisms break it down through aerobic and anaerobic decomposition, which is the principle behind activated sludge and biogas-based treatment in sewage treatment plants.

Why the others fail the test of biodegradability:

• A) Mercury is an element. Microbes cannot break it down, so it persists and bioaccumulates and biomagnifies in food chains, as in Minamata disease. Toxic does not mean biodegradable.
• C) Plastic is made of synthetic polymers such as polyethylene that resist microbial action and persist for decades or longer. The Plastic Waste Management Rules, 2016, and their later amendments address this.
• D) Asbestos is a silicate mineral fibre that does not decompose. It causes asbestosis and mesothelioma, and its use has been progressively restricted.

Mains angle: this links to municipal and liquid waste management, and to river pollution programmes such as Namami Gange, where sewage treatment is the central intervention.

Trap: candidates pick mercury because it is the most hazardous of the four. The question asks about degradability by biological agents, not toxicity.