The correct choice is (a) Lindane
The explanation is: Lindane is the toxic compound that can be degraded by Sphingomonas paucimobilis, and it can effectively degrade it into simple compounds which are not harmful to the environment. Also, Sphingomonas paucimobilis can effectively degrade HCH i.e. Hexachlorocyclohexane molecule which is known to be more toxic. Carbon dioxide, Lithium and Magnesium are not toxic to the environment as Lindane.