Which of the following is a double salt ?
Alum
Chrome alum
(c ) Microcosmic salt
(d ) All of these .

Explanation

To determine which of the following is a double salt, we need to analyze each option based on the definition of double salts. A double salt is a compound that contains more than one type of cation or anion and dissociates into its constituent ions when dissolved in water.

### Step-by-Step Solution:

1. **Understanding Double Salts**:
- A double salt is a combination of two different salts that crystallize together and can dissociate into their respective ions when dissolved in water.

2. **Analyzing Alum**:
- Alum has the general formula \( A_2SO_4.B_2(SO_4)_3.24H_2O \).
- When dissolved in water, it dissociates into \( 2A^+ + 3B^{3+} + 4SO_4^{2-} + 24H_2O \).
- Since it dissociates into ions, alum is a double salt.

3. **Analyzing Chrome Alum**:
- The formula for chrome alum is \( K_2SO_4.Cr_2(SO_4)_3.24H_2O \).
- Upon dissolution, it dissociates into \( 2K^+ + 2Cr^{3+} + 3SO_4^{2-} + 24H_2O \).
- This also shows that chrome alum dissociates into ions, confirming it is a double salt.

4. **Analyzing Microcosmic Salt**:
- The formula for microcosmic salt is \( NaNH_4HPO_4.4H_2O \).
- When it dissolves, it dissociates into \( Na^+ + NH_4^+ + HPO_4^{2-} + 4H_2O \).
- This dissociation indicates that microcosmic salt is also a double salt.

5. **Conclusion**:
- Since all three compounds (Alum, Chrome Alum, and Microcosmic Salt) dissociate into their respective ions when dissolved in water, they all qualify as double salts.
- Therefore, the correct answer is (d) All of these.

### Final Answer:
**(d) All of these.**