A double salt is a compound that contains two different cations or anions. By examining the options provided, we can analyze each one to determine if it fits the criteria for a double salt.

Option 1, K4[Fe(CN)6], does not fit the definition of a double salt because it only contains one type of cation, potassium (K+).

Option 2, K2S04. A12(S04)3.24H20, is the correct answer. It contains both the cation potassium (K+) and the cation aluminum (Al3+), making it a double salt. Additionally, it also contains the sulfate anion (S042-).

Option 3, CuS04.5H20, is not a double salt because it only contains one type of cation, copper (Cu2+), and the sulfate anion (S042-).

Option 4, NaCl, is not a double salt because it only contains one type of cation, sodium (Na+), and one type of anion, chloride (Cl-).

In conclusion, option 2, K2S04. A12(S04)3.24H20, is the only option among the provided choices that