Final answer:
The action that is a priority in this situation is to obtain a serum glucose level. Monitoring the glucose level will help determine the appropriate course of action, such as administering glucose or glucagon.
Step-by-step explanation:
The action that is a priority in this situation is A. Obtain a serum glucose level. When a patient with type 1 diabetes mellitus is found unresponsive with cool and clammy skin, it is crucial to measure their blood glucose levels to assess for hypoglycemia. Hypoglycemia can be a life-threatening condition in patients with type 1 diabetes, and prompt treatment is necessary to raise the blood glucose levels to a safe range. Monitoring the glucose level will help determine the appropriate course of action, such as administering glucose or glucagon.