Natural rubber is:

Options

A. All-trans polyisoprene
B. Chloroprene
C. Buna-`S`
D. All-cis polyisoprene

Answer

### Step-by-Step Solution:

1. **Identify the Composition of Natural Rubber**:
- Natural rubber is primarily composed of a polymer known as polyisoprene.

2. **Determine the Type of Polyisoprene**:
- Polyisoprene can exist in different forms, but natural rubber specifically consists of the cis form of polyisoprene, referred to as cis-1,4-polyisoprene.

3. **Understanding the Structure of Isoprene**:
- The monomer unit of natural rubber is isoprene, which has the chemical structure represented as CH2=C(CH3)-CH=CH2.

4. **Formation of Natural Rubber**:
- When multiple isoprene units (n moles) polymerize, they form the long-chain structure of natural rubber, which is the cis-1,4-polyisoprene.

5. **Properties of Natural Rubber**:
- Natural rubber is characterized as a linear, unsaturated, long-chain aliphatic hydrocarbon polymer. It possesses important mechanical properties such as softness and elasticity.

6. **Conclusion**:
- Based on the above information, we conclude that natural rubber is essentially cis-1,4-polyisoprene.

### Final Answer:
Natural rubber is **cis-1,4-polyisoprene**.