The correct answer is D) suspension of breathing. When providing first aid to a victim of gas poisoning, the most important symptom to check for is the suspension of breathing, as gas poisoning can lead to respiratory failure. Maintaining an open airway and ensuring the victim is breathing properly is the top priority in these situations. The other options, such as a slow and weak pulse, cold and moist skin, or unconsciousness, are important to monitor, but the suspension of breathing is the most critical symptom that requires immediate attention and intervention.