Biodegradable wastes can usually be converted into useful substance with the help of

Correct answer: (a) bacteria

Explanation

Biodegradable wastes (wood, fruits, vegetables, dead plants and animals) are broken down by microbes, chiefly bacteria, through composting and aerobic/anaerobic digestion.

This converts organic matter into useful simpler substances like carbon dioxide, methane and water; viruses and radioactive substances play no such role.