To determine which of the given options is a true solid, we need to analyze each option based on the characteristics of true solids, which are typically crystalline in nature. True solids have a fixed shape, are rigid, possess a sharp melting point, and are highly dense.
### Step-by-Step Solution:
1. **Understanding True Solids**:
- True solids, or crystalline solids, exhibit a definite and orderly arrangement of particles. They have distinct properties such as fixed shape, rigidity, sharp melting points, and high density.
2. **Analyzing Sodium Chloride (NaCl)**:
- Sodium chloride is an ionic compound composed of Na⁺ and Cl⁻ ions.
- It forms a crystalline structure, which means it has a fixed shape and is rigid.
- Sodium chloride has a sharp melting point and is highly dense.
- Therefore, sodium chloride qualifies as a true solid.
3. **Analyzing Glass**:
- Glass is often considered a supercooled liquid rather than a true solid.
- It does not have a fixed shape and can flow over time, which means it lacks the rigidity characteristic of true solids.
- Thus, glass is not a true solid.
4. **Analyzing Rubber**:
- Rubber is classified as an amorphous solid.
- It is compressible and does not maintain a fixed shape, which indicates it is not rigid.
- Therefore, rubber does not qualify as a true solid.
5. **Analyzing Polythene**:
- Polythene is a polymer that exhibits properties of flexibility and stretchability.
- It does not have a fixed shape and can be deformed easily, indicating it is not rigid.
- Thus, polythene is also not a true solid.
6. **Conclusion**:
- After analyzing all the options, the only material that qualifies as a true solid is sodium chloride (NaCl).
### Final Answer:
The true solid among the given options is **Sodium Chloride (NaCl)**.
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