Which scenario mandates a prompt concussion evaluation and a 24-hour rest period?

The scenario that mandates a prompt concussion evaluation and a 24-hour rest period is when any soldier is within 50 meters of a blast. This is critical because exposure to blasts can result in concussive injuries, even if the individual does not lose consciousness or show immediate symptoms. The impact of the blast pressure wave can affect the brain, leading to potential concussions.

In this context, the 24-hour rest period is important as it allows for proper monitoring and recovery. Symptoms of concussion may not always be apparent right away, and the rest period helps in assessing the soldier's condition thoroughly and ensures that they do not undertake any activities that could further aggravate a potential injury.

The other choices present scenarios that do not necessarily trigger the same level of concern regarding concussive injuries as being within proximity to an explosion does. A simple headache could arise from many different causes and does not inherently indicate a concussion, while soldiers at rest in a vehicle or those not involved in combat would generally be at lower risk for concussion due to less exposure to potentially harmful explosive forces.