Among the following textile fibres, which one is a natural animal derived fibre rather than a man made or synthetic fibre?

Introduction / Context:
Fibres used in textiles can be classified broadly into natural fibres and man made fibres. Natural fibres are obtained directly from plants or animals, whereas synthetic fibres are produced by chemical processes in factories. Understanding these categories is important in materials science and everyday knowledge of fabrics. This question asks you to identify the natural fibre among commonly used textile materials.

Given Data / Assumptions:

• Silk is produced by silkworms, which are animals.
• Rayon is made from chemically processed cellulose and is often called semi synthetic.
• Nylon and polyester are fully synthetic petrochemical based fibres.
• We focus on whether the fibre source is directly natural or man made.

Concept / Approach:
Silk is a natural animal fibre produced by the larvae of certain moths, especially the mulberry silkworm. The fibre is spun into cocoons and harvested for weaving into cloth. Rayon is made by chemically treating cellulose from wood pulp, so it is considered a regenerated or semi synthetic fibre rather than entirely natural in its final form. Nylon and polyester are fully synthetic polymers made by chemical reactions from petroleum products. Therefore, among the given options, silk is the only natural fibre in the traditional sense.

Step-by-Step Solution:
Step 1: Recall that natural fibres include cotton, wool, silk, jute and similar materials. Step 2: Recognise that silk is obtained directly from silkworm cocoons and is classified as a natural animal fibre. Step 3: Understand that rayon is produced by chemically processing plant cellulose and is categorised as a man made regenerated fibre. Step 4: Note that nylon and polyester are synthetic fibres formed from petrochemicals and do not occur in nature. Step 5: Conclude that silk is the correct choice as the natural fibre.

Verification / Alternative check:
Textile and science textbooks list fibres under two headings: natural and man made. Under natural, they include cotton from plants and wool and silk from animals. Man made fibres include rayon as regenerated cellulose and synthetic fibres like nylon, polyester and acrylic, all produced in chemical factories. This classification appears consistently in educational material and confirms that silk is the natural fibre among the options.

Why Other Options Are Wrong:
Option b, rayon, starts from natural cellulose but is heavily processed and classified as a man made fibre in standard school curricula.

Option c, nylon, is a fully synthetic polymer invented by chemists and does not exist in nature.

Option d, polyester, is another synthetic polymer widely used in fabrics and is produced from petrochemicals.

Common Pitfalls:
Students sometimes believe that rayon must be natural because it originates from wood pulp, or they think that any fibre that feels like cotton or silk must be natural. Others may consider the softness or comfort of a fabric and use that as a guide. However, classification is based on the origin and manufacturing process rather than feel. Remembering the main examples in each category helps avoid confusion in such questions.

Final Answer:
Among the listed fibres, Silk is the natural animal derived fibre.