Which one of the following four metals would be displaced from the solution of its salts by the other three metals?

Ag would be displaced from its salt solution by other metals. 
As silver is a non-reactive metal which is present in its native state it can be easily displaced by other metals. 
Ag can easily be displaced by Cu, Mg, and Zn from the solution of its salt.

Due to the high standard reduction potential Ag⁺ ion has the highest oxidising strength among all the metals (ions). All the metal ions lose electrons while Ag⁺ ion gains an electron.