Drying agent which reacts with `CO_2` and involves water vapour is

Options

A. CaO
B. `CaCl_2`
C. `CaCO_3`
D. `Ca(NO_3)_(2)`

Answer

To solve the question regarding the drying agent that reacts with CO2 and involves water vapor, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Drying Agent**:
- A drying agent is a substance that removes moisture from the air or other substances. Common drying agents include calcium oxide (CaO), magnesium sulfate (MgSO4), and silica gel.

2. **Consider the Reaction with CO2**:
- Among the common drying agents, calcium oxide (CaO) is known to react with carbon dioxide (CO2). The reaction can be represented as:
\[ \text{CaO} + \text{CO}_2 \rightarrow \text{CaCO}_3 \]
- Here, calcium oxide reacts with carbon dioxide to form calcium carbonate (CaCO3).

3. **Involvement of Water Vapor**:
- Calcium oxide also reacts with water to form calcium hydroxide (Ca(OH)2):
\[ \text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 \]
- When CO2 is present in the air, it can react with the water vapor to form carbonic acid, which can further react with calcium hydroxide to form calcium carbonate.

4. **Conclusion**:
- Therefore, calcium oxide (CaO) is the drying agent that reacts with CO2 and involves water vapor in its reactions.

### Final Answer:
The drying agent which reacts with CO2 and involves water vapor is **Calcium oxide (CaO)**.