Explanation:

When a more reactive metal displaces a less reactive metal from its salt solution, it is known as a displacement reaction. In this case, we need to determine which metal can displace the other three metals from their salt solutions.

To determine the reactivity of the metals, we can refer to the reactivity series of metals. The reactivity series is a list of metals arranged in order of their reactivity, with the most reactive metal at the top and the least reactive metal at the bottom.

The reactivity series of metals from most reactive to least reactive is as follows:
Potassium (K) - Sodium (Na) - Calcium (Ca) - Magnesium (Mg) - Aluminum (Al) - Zinc (Zn) - Iron (Fe) - Lead (Pb) - Hydrogen (H) - Copper (Cu) - Silver (Ag) - Gold (Au)

Analysis of the options:
a) Cu (Copper) - Copper is less reactive than silver, magnesium, and zinc. It cannot displace any of the other three metals from their salt solutions.

b) Ag (Silver) - Silver is less reactive than magnesium and zinc but more reactive than copper. It can displace copper from its salt solution but cannot displace magnesium or zinc.

c) Mg (Magnesium) - Magnesium is more reactive than copper, silver, and zinc. It can displace copper and silver from their salt solutions but cannot displace zinc.

d) Zn (Zinc) - Zinc is more reactive than copper, silver, and magnesium. It can displace copper, silver, and magnesium from their salt solutions.

Conclusion:
Based on the reactivity series, the metal that can be displaced from the solution of its salts by the other three metals is silver (Ag). Therefore, the correct answer is option b) Ag.