A perfectly soldered joint will show the entire cup area of the fitting to be completely covered with what?

A perfectly soldered joint in plumbing requires that the entire cup area of the fitting be completely covered with solder. Solder is a filler metal that melts when applied and then solidifies to create a strong, durable bond between two metal surfaces, in this case, copper pipes and fittings.

When the solder is applied correctly, it flows into the joint and completely fills the annular space between the pipe and the fitting, ensuring that there are no gaps or leaks. The molten solder is drawn into the joint by capillary action, and it solidifies as it cools, creating a watertight seal.

Other options, such as flux, play an important role in the soldering process by cleaning and preventing oxidation of the surfaces, but they do not form the final joint. Copper refers to the base material of the pipes and fittings, while adhesive is not typically used in soldering joints involving metal. Hence, to achieve a perfectly soldered joint, the presence of solder is what indicates a proper and effective seal.