What type of solder should be used when soldering two #20 AWG copper wires together?

Using rosin core solder is appropriate for soldering two #20 AWG copper wires together because rosin core solder contains a flux that helps to clean and prepare the metal surfaces for soldering. The flux works by removing oxidation and preventing further oxidation during the soldering process, which ensures a strong and reliable electrical connection.

Rosin core solder is widely recommended for electrical applications due to its effectiveness in providing good wetting properties on copper wires, resulting in a solid bond. The rosin insulation makes the solder safe to use in electronics as it is non-corrosive and doesn't cause issues with electrical conductivity once it cools and solidifies.

While other forms of solder, such as lead-free, may be used for general soldering purposes, the specific requirement for a conductive and reliable solder joint in electrical work makes rosin core solder the best choice in this scenario.