The correct answer is option 3: Pb and Sn.

Solder is an alloy that is used to join or bond metal surfaces together. It is a low melting point alloy, which makes it suitable for applications in electronics, plumbing, and metalwork.

Option 1, which suggests that solder is composed of copper (Cu) and tin (Sn), is incorrect. While copper and tin are both used in the manufacturing of solder, they are not the only components.

Option 2, which suggests that solder is composed of iron (Fe) and zinc (Zn), is also incorrect. Iron and zinc do not commonly form an alloy called solder.

Option 4, which suggests that solder is composed of silver (Ag) and zinc (Zn), is incorrect as well. Although silver-based solders do exist, they are not commonly used and are more expensive than other types of solder.

Therefore, the correct alloy for solder is a combination of lead (Pb) and tin (Sn). This mixture provides the desired low melting point and suitable properties for joining metals efficiently.