When `Na_2CO_3 . 10 H_2O` is kept open in air, it
(a)absorbs moisture
(b)loses water to form anhydrous salt
(c) loses water to form a monohydrate
(d) decomposes to give `CO_2 and Na_2O`.

Options

A. absorbs moisture
B. loses water to form anhydrous salt
C. loses water to form a monohydrate
D. decomposes to give `CO_2 and Na_2O`.

Explanation

To solve the question regarding the behavior of `Na2CO3 . 10 H2O` (sodium carbonate decahydrate) when kept open in air, we can analyze the options step by step.

### Step-by-step Solution:

1. **Understanding Sodium Carbonate Decahydrate**:
- Sodium carbonate decahydrate, represented as `Na2CO3 . 10 H2O`, is commonly known as washing soda. It contains 10 molecules of water of crystallization.

2. **Behavior in Open Air**:
- When `Na2CO3 . 10 H2O` is exposed to air, it can undergo a process known as efflorescence. This is the loss of water of crystallization due to evaporation.

3. **Loss of Water**:
- As it loses water, it does not lose all 10 molecules at once. Instead, it typically loses several water molecules. In this case, it loses 9 molecules of water.

4. **Formation of Monohydrate**:
- After losing 9 water molecules, the remaining compound is `Na2CO3 . H2O`, which is sodium carbonate monohydrate. This indicates that it has transformed from a decahydrate to a monohydrate.

5. **Evaluating the Options**:
- (a) Absorbs moisture: This is incorrect because it is losing water, not absorbing it.
- (b) Loses water to form anhydrous salt: This is incorrect because it does not lose all water to become anhydrous; it retains 1 water molecule.
- (c) Loses water to form a monohydrate: This is correct as it loses 9 water molecules to form `Na2CO3 . H2O`.
- (d) Decomposes to give `CO2` and `Na2O`: This is incorrect; there is no decomposition into these products.

6. **Conclusion**:
- The correct answer is (c) loses water to form a monohydrate.

### Final Answer:
(c) loses water to form a monohydrate.