The correct answer is D) cover the burns with thick sterile dressing. For third-degree burns, which are the most serious, the key is to avoid breaking blisters, removing dead tissue, or immersing the burn area in water, as these actions can further damage the skin and increase the risk of infection. Instead, the recommended treatment is to cover the burn area with a thick, sterile dressing to protect it, reduce pain, and prevent further heat loss or contamination. This is the appropriate first aid measure for third-degree burns, as specified in Coast Guard regulations and standard medical protocols. The other options, such as making the person stand up or break blisters, would be incorrect and potentially harmful for treating third-degree burns.