This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
The hormone using an enzyme receptor for its action: 
Select and explain. 
A. Insulin, B. Steroid, C. Oestrogen, D. Thyroxine
A: Insulin
Answer: A: Insulin 
Explanation: A i.e. Insulin