Which of the following is not a natural fibre ?

Rayon 

The correct answer is option 3. Rayon.

Rayon is not a natural fiber. It is a semi-synthetic or artificial fiber. While it is derived from natural cellulose, usually from wood pulp, it undergoes extensive processing with chemicals, making it a manufactured fiber.

The other options:

Linen is a natural fiber derived from the flax plant.

Cotton is a natural fiber obtained from the cotton plant.

Jute is a natural fiber obtained from the jute plant.

Therefore, the correct answer is: Rayon