Final answer:
Upon an adult becoming unresponsive during abdominal thrusts for severe choking, you should start CPR and call for emergency services.
Step-by-step explanation:
If a choking adult becomes unresponsive while you are doing abdominal thrusts for severe choking, the correct action would be to begin CPR (Cardiopulmonary Resuscitation). As soon as the person becomes unresponsive, you should call out for someone to dial emergency services (activate the emergency response system) if it hasn't been done already. If no one else is available, do this yourself immediately after starting CPR.
When administering CPR, remember to perform chest compressions and rescue breaths at the correct ratio and depth. Care must be taken to ensure that CPR is performed correctly to avoid causing harm such as broken ribs or damage to the liver through incorrect hand positioning.