Division 4.3 materials are characterized as:

Answer: B — Dangerous when wet

Division 4.3 materials are classified as "dangerous when wet" because they include substances that can react with water to produce flammable gases or other hazardous materials. This classification highlights the significant risks posed when these materials come into contact with moisture, which may lead to fires, explosions, and toxic gas releases. Understanding the behavior of these materials in various conditions, particularly their reactivity to water, is crucial for safe handling and transportation in the trucking industry. Ensuring all safety protocols around these materials are followed is essential to prevent accidents and ensure public safety.