Step 1: Understanding the Concept:
Polymers are grouped by the strength of forces between their chains. This gives elastomers, fibres, thermoplastics and thermosetting plastics.
Step 2: Detailed Explanation:
Natural rubber is cis-1,4-polyisoprene. Its chains are held by weak van der Waals forces and are coiled, so it stretches and then returns to its shape. Polymers that behave like this are elastomers.
Step 3: Why the other options are wrong.
Fibres have strong hydrogen bonds or dipole forces (nylon). Thermoplastics soften on heating (polythene). Thermosetting plastics harden on heating (bakelite).
Final Answer:
Natural rubber is an elastomer, option (B).
\[ \boxed{\text{Elastomer}} \]