Which of the following group is not 'biodegradable'?

As the chapter explains, substances that are broken down by biological processes (the action of bacteria and other saprophytes/microorganisms) are called biodegradable, while those that resist such breakdown are non-biodegradable. This is because enzymes are specific in their action — the same enzymes that digest natural organic matter cannot act on synthetic, human-made materials.

• Option A (Vegetable peels, dead leaves, paper): All three are natural, plant-derived organic materials that decomposers readily break down — this is a purely biodegradable group, so it is not the answer.
• Option B (Cow dung, leather bag, water): Cow dung and leather (an animal-derived organic material) are biodegradable, and water is not 'waste' in this biodegradable/non-biodegradable sense at all — none of these three is a persistent synthetic pollutant, so this group is not the correct choice either.
• Option C (Polythene bag, rubber band, ball pen): Polythene (a plastic), rubber, and the plastic/ink components of a ball pen are all synthetic, human-made materials. As stated in the text, 'many human-made materials like plastics will not be broken down by the action of bacteria or other saprophytes... under the ambient conditions found in our environment, these persist for a long time.' Hence this entire group is non-biodegradable — this is the correct answer.
• Option D (Paper, fruits, bones): Paper (derived from wood pulp), fruits, and bones are all organic and are readily decomposed by microorganisms, making this group biodegradable, not the answer.