What is a risk when welding on closed containers?

When welding on closed containers, a significant risk is the potential for an explosion, making this the correct answer. Closed containers can trap gases or vapors that are flammable or explosive. When an electric arc or heat from welding occurs, it can ignite these trapped gases, leading to a sudden and violent explosion. This is especially concerning in industries that deal with volatile substances, as the hazards escalate without proper ventilation or safety measures.

Addressing the other choices, while exposure to harmful vapors is indeed a risk when welding (making the option regarding exposure relevant), it is not as immediate or severe as the risk of explosion. The suggestion that it reduces the strength of the metal is somewhat misleading; welding, when done correctly, is intended to join materials and may actually enhance structural integrity. The option asserting that it protects employees from injuries does not accurately reflect the reality of welding in confined spaces, where risks tend to be higher rather than lower. Understanding the significant danger of explosions is crucial for ensuring safety protocols are followed when working with closed containers.