Which of the following is a natural fibre? 

Wool 

The correct answer is option 1. Wool.

Out of the options you listed, the natural fiber is: Wool

Wool is a natural protein fiber derived from the fleece of sheep, goats, and some other animals. It has been used for textiles and clothing for thousands of years due to its warmth, softness, and other desirable properties. Nylon, PVC, and Polythene are all synthetic fibers.

These are all man-made polymers created from chemical processes, not derived from natural sources like plants or animals.