What materials primarily make up solder?

Solder primarily consists of tin and lead, which is why this choice is correct. These two metals have been traditionally used in solder formulations due to their favorable melting points and excellent flow characteristics when heated, which allow for a strong bond in electrical and plumbing applications. The combination of tin and lead provides a reliable joining method for various materials, especially in electronics.

While other materials like gold and silver can indeed be used in solder applications, they are typically utilized for specialized or high-end applications due to their cost and properties. Copper and zinc are more commonly found in alloys and other applications, but they do not form the standard solder used for most general purposes. Aluminum and iron, though important in different contexts, are not suitable for soldering applications as they do not behave like traditional solder materials and are subject to oxidation, which interferes with the soldering process.