Classification (materials – origin): Three are synthetic polymers; one is a natural protein fibre. Identify the odd one out.

Introduction / Context:
Materials classification often hinges on source and composition. Nylon, polythene (polyethylene), and generic plastics are man-made synthetic polymers derived from petrochemical processes. Silk, in contrast, is a natural protein fibre produced by silkworms. The natural vs synthetic origin creates the standard odd-one-out boundary used in many reasoning exams.

Given Data / Assumptions:

• Nylon → synthetic polymer.
• Polythene → synthetic polymer (polyethylene).
• Plastic → umbrella term generally referring to synthetic polymers.
• Silk → natural animal fibre (protein).

Concept / Approach:
Group items by origin (natural vs synthetic). With exactly one natural item present, the selection is straightforward.

Step-by-Step Solution:

Verification / Alternative check:
Ask which item is produced biologically rather than industrially; silk alone fits this description.

Why Other Options Are Wrong:

Common Pitfalls:
Equating “plastic” with a specific polymer. The generic label does not change its synthetic nature.

Final Answer:
Silk