Generally transition elements form coloured salts due to the presence of unpaired electrons. Which of the following compounds will be coloured in solid state?

Electronic configuration of

Ag⁺ - [Kr] 4d¹⁰

Cu²⁺ - [Ar] 3d⁹

Zn^{2+ -} [Ar] 3d¹⁰

Cu⁺ - [Ar] 3d¹⁰ 4s¹

Among the four ions, only Cu²⁺ has an unpaired electron in the d orbital which will give rise to coloured compounds.