Concept:Biodegradable pollutants are wastes that can be broken down by natural biological processes.Explanation:Sewage contains organic matter and microorganisms that decompose it naturally. Plastic is non-biodegradable because it resists microbial breakdown. Carbon (II) oxide and hydrogen sulphide are gases, not biodegradable wastes. Therefore, among the given options, sewage is the biodegradable pollutant.Answer:B. sewage