Which of the following is considered a byproduct of lead?

Silver is identified as a byproduct of lead due to the characteristics of mining and mineral extraction processes. In certain lead ores, silver can naturally occur alongside lead and is often extracted during the smelting or refining stages of lead production. The mining operations specifically targeting lead may unintentionally yield silver as well, hence its classification as a byproduct.

In contrast, copper, gold, and iron do not typically occur with lead in a way that classifies them as byproducts. While they are valuable metals, they are extracted from their respective ores, and their relationship with lead is not as interconnected in terms of the mining and processing stages. Thus, silver stands out as the correct answer in this context.