Which materials must be kept dry while in transit and during the loading and unloading process?

The correct answer focuses on the need to keep materials that belong to Class 4 (flammable solids) and Class 5 (oxidizers) dry because moisture can cause these substances to react in dangerous ways. Flammable solids can ignite when in contact with moisture, and oxidizers can enhance combustion and create fire hazards if they come in contact with water. Therefore, maintaining a dry environment during transit and handling is critical to ensure safety and prevent accidental reactions that could lead to fires, explosions, or other dangerous incidents involving these materials.

While solid materials are mentioned in another option, not all solids pose the same risks as flammable solids and oxidizers. Similarly, Class 6 materials primarily involve toxic substances, which also require careful handling but may not necessarily be affected by moisture in the same way. Non-hazardous items, as stated in the last option, do not require special precautions regarding moisture and thus are not relevant to the question about materials that must remain dry. Therefore, focusing on the specific risks associated with flammable solids and oxidizers justifies why keeping them dry during transport and handling is of utmost importance.