Step 1: Understanding the Concept
Fibres are classified as natural (vegetable or animal), semisynthetic and synthetic.
Step 2: Silk
Silk is spun by silkworms and is a protein called fibroin.
A fibre of animal origin is called an animal fibre, like wool.
Step 3: Compare
Cotton and jute are vegetable fibres, rayon is semisynthetic, and nylon is synthetic.
So silk is an animal fibre.
Final Answer:
Silk is an animal fibre.
\[ \boxed{\text{(C) }\text{Animal}} \]