For mild and serious symptoms of DCS and overexpansion injuries, the appropriate first aid is:

The correct response highlights the comprehensive nature of first aid required for both mild and serious symptoms of decompression sickness (DCS) and overexpansion injuries. Administering 50% oxygen can help alleviate symptoms and support recovery by providing the body with the necessary oxygen levels, which is vital in such cases. Keeping the victim calm is equally important, as stress can exacerbate symptoms and impact the body's ability to recover. Monitoring vital signs is crucial for assessing the victim's condition and determining if further medical intervention is required.

By stating that all answers are correct, the response acknowledges that each action plays a vital role in effective first aid management, emphasizing a multidimensional approach to treating DCS and overexpansion injuries. This approach is essential to ensure the best outcomes for the victim, making it clear that all provided measures should be integrated into the first aid response instead of relying on isolation of a single action.