1. Recall the definition of a double salt

A double salt is formed by crystallization of two simple salts in a definite stoichiometric ratio.

• In solid state, it exists as a single crystalline compound.
• In aqueous solution, it dissociates completely into all the constituent ions.

Examples: Mohr’s salt, alum, carnallite.

Compounds that do not dissociate into all constituent ions in solution are generally coordination compounds / complex salts, not double salts.

2. Examine each given compound

(A) $\mathrm{FeSO}_{4} \cdot (\mathrm{NH}_{4})_{2}\mathrm{SO}_{4} \cdot 6\mathrm{H}_{2}O$

This is Mohr’s salt.

It is a well-known double salt formed from:

• $\mathrm{FeSO_4}$
• $(\mathrm{NH_4})_2\mathrm{SO_4}$

Hence, A is a double salt.

(B) $\mathrm{CuSO}_{4}\cdot 4\mathrm{NH}_{3}\cdot \mathrm{H}_{2}O$

This is better written as a complex like tetraammine copper(II) sulfate hydrate.

It contains the complex ion: $[\mathrm{Cu}(\mathrm{NH}_3)_4]^{2+}$ with sulfate as counter ion.

Since it behaves as a coordination compound (complex salt) and not as a double salt, B is not a double salt.

(C) $\mathrm{K}_{2}\mathrm{SO}_{4} \cdot \mathrm{Al}_{2}(\mathrm{SO}_{4})_{3} \cdot 24\mathrm{H}_{2}O$

This is potash alum.

Alums are classic double salts. In water, they dissociate into: $2\mathrm{K}^+, \; 2\mathrm{Al}^{3+}, \; 4\mathrm{SO}_4^{2-}$

Hence, C is a double salt.

(D) $\mathrm{Fe}(\mathrm{CN})_{2} \cdot 4\mathrm{KCN}$

This is equivalent to: $\mathrm{K}_4[\mathrm{Fe}(\mathrm{CN})_6]$ which is potassium ferrocyanide, a complex salt.

It contains the complex ion: $[\mathrm{Fe}(\mathrm{CN})_6]^{4-}$

Therefore, D is not a double salt.

3. Select the correct set

Double salts among the given compounds are:

• A: Yes
• B: No
• C: Yes
• D: No

So the correct choice is: $\boxed{\text{A and C only}}$

This corresponds to Option D.

4. Comparison with stored correct answer

Stored correct answer: D

Our derived answer: D

They match.