The right option is; b. oil absorption
Oil absorption is not a way in which wetlands filter water.
Wetlands are areas that are covered mostly with water, either periodically or permanently, and are occasionally marshy and saggy areas. Wetlands functions like a strainer by filtering out harmful substances and excess nutrients in water. Wetland soils also trap natural sediment and absorb bacteria and microorganisms which decompose harmful substances in water, thereby making the substances less harmful.