Some salts containing two different metallic elements give test for only one of them in solution, such salts are:
A.Double salts
B.Normal salts
C.Complex salts
D.None of these

Hint:Complex salts are those in which there is a central metal ion that forms a co-ordinate bond with ligands by accepting the electron pairs from them. These coordinate bonds are strong as the ligands satisfy the secondary valence of the metal ions.

Complete answer:
Complex salts are formed when the secondary valences of the transition metal atoms are satisfied by ligands that donate electron pairs. These are called the coordination compounds. The bond between the metal and the ligands are strong bonds that cannot be broken easily, hence if there are two different metallic elements, one maybe a complex ion that does not give a test for only one of them in solution.

Thus option C, complex salts is correct.

Note:
Among the above given solutions, the double salts are those that have the composition of a mixture of two simple salts but with a different crystal structure is called a double salt. These salts dissociate in the solution to readily liberate the cations and the anions that they are composed of. Normal salts are made of one cation and one anion that are in close contact with each other due to the ionic attraction between them, these salts also dissociate readily in water if the hydration energy of the cations and the anions is higher than the lattice energy of the salts. The normal salts are generally formed by the reaction of an acid with a base.