Which of the following compounds is efflorescent ?

Options

A. A. `Na_2CO_3`
B. B. `Na_2CO_3 . 10H_2O`
C. C. `Na_2CO_3 . H_2O`
D. D.`NaHCO_3`

Explanation

To determine which of the following compounds is efflorescent, we need to understand the definition of efflorescence. Efflorescent compounds are those that lose their water of crystallization when exposed to the atmosphere, typically due to evaporation.

### Step-by-Step Solution:

1. **Understand the Definition of Efflorescent Compounds**:
- Efflorescent compounds are hydrated salts that lose water of crystallization upon exposure to air. This process occurs naturally and does not require extreme conditions.

2. **Identify Common Efflorescent Compounds**:
- Some common efflorescent compounds include sodium carbonate decahydrate (Na2CO3·10H2O), copper(II) sulfate pentahydrate (CuSO4·5H2O), and others.

3. **Analyze the Given Compounds**:
- Without the specific list of compounds provided in the question, we can focus on the example of sodium carbonate decahydrate (Na2CO3·10H2O). This compound is known to lose water of crystallization and become anhydrous sodium carbonate (Na2CO3) when exposed to air.

4. **Conclusion**:
- Based on the understanding that sodium carbonate decahydrate loses water of crystallization upon exposure to air, we can conclude that it is an efflorescent compound.

5. **Final Answer**:
- The efflorescent compound among the options provided is sodium carbonate decahydrate (Na2CO3·10H2O).