Which of the following is NOT a leading cause of lead contamination?

The choice indicating proper storage of metal containers as not being a leading cause of lead contamination is correct because the way metal containers are stored does not typically contribute to lead contamination in the environment. Properly maintained and stored metal containers can actually prevent lead exposure by ensuring that toxic substances do not spill or leak into the soil or water sources.

In contrast, industrialization, soil contamination, and unregulated chemical dumping are significant contributors to lead contamination. Industrial processes often involve the use of lead, whether it's in manufacturing or as a byproduct, leading to environmental contamination. Soil contamination can occur from various sources, including lead-based paints and industrial waste, resulting in lead being present in the soil where it can be ingested or inhaled. Unregulated chemical dumping often includes hazardous materials, including lead, which can leach into the ground and water, thus contaminating local ecosystems. Therefore, the focus on proper storage practices highlights a preventative measure rather than a cause of contamination.