Why is rubber so elastic?

Hint: Rubber is a polymer made of linking the same type of large number of molecules. It consists of cross linking between the chains. Rubber is also known as an elastomer. The polymeric chains present in rubber are so long that they tangle into each other and get twisted and coiled.

Complete answer:
Rubber is a polymer which is made up of linking of molecules through chemical bonds. They consist of long chain-like molecules made up of small repeating units. The molecules of rubber are so long that they bend and coil randomly. Therefore, a rubber is made of vast numbers of twisted, rope like molecules.
When the rubber is pulled, the twists and coils present in its molecules are straightened out and hence a restoring force is developed in it due to which when the stress is removed, the twists and coils are formed again in the molecules and the rubber returns to its original shape.
The modulus of elasticity is the ratio of stress upon strain and gives the measure of a material’s elasticity. Rubber has a small value of modulus of elasticity
Therefore, the rubber is elastic because it contains long rope like twisted and coiled molecules which are easily stretched.

Note:
The stress is the force applied per unit area while the strain is the relative change in the dimensions of a body on the action of a stress. The modulus of elasticity gives us a measure of the resistive force applied by a body on being stretched. It is a unitless quantity.