The correct answer is bacteria.
The decomposition is the process by which the bacteria and the other microorganism, breakdown the large molecules into smaller organic/inorganic component. these smaller components can be easily absorbed and mixed with the soil.
Naturally occurring bacteria generally cannot survive in toxic environment. by genetic engineering, scientists can introduce the gene, which can help them to survive in toxic conditions.
Genetically engineered bacteria can be helpful in decomposing the toxic waste.