The correct answer is option 2 - Pb and Sn.

An alloy solder is a material used for joining metallic surfaces together, typically in soldering or brazing processes. It is composed of two or more metals, which are melted together and used to create a strong bond between the surfaces being joined.

In the case of alloy solder, the two main constituents are lead (Pb) and tin (Sn). This combination of metals provides the desired properties for soldering, such as low melting point, good wetting ability, and the ability to form a strong bond with different types of metals.

Option 1 (Pb and Zn) is incorrect as zinc is not commonly used as a constituent in alloy solder. Similarly, option 3 (Pb and Si) is incorrect as silicon is not typically used in solder alloys. Option 4 (Pb and Co) is also incorrect as cobalt is not commonly used in soldering alloys.

It is important to note that there are different types of alloy solders available, and the specific constituents may vary depending on the desired properties and application.