Colourless solution of the following four salts are separately in four different test tube and a strip of copper is dipped in each one of these . Which solution will turn blue?

Copper solution will become blue when ${\mathrm{AgNO}}_3$ is added. The production of ${\mathrm{Cu}}^{2+}$ ions is what gives the colour blue. In the electrochemical hierarchy, silver comes before copper. Copper is positioned below $\mathrm{Zn}$ and $\mathrm{K}$in the electrochemical series. As a result, copper has the ability to remove silver ions from its salt solution while $\mathrm{K}$ and $\mathrm{Zn}$ are unable to do so, ${\mathrm{AgNO}}_3$.