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: Biochemistry
Which is an allosteric protein: 
Select and explain. 
A. Transferrin, B. Ceruloplasmin, C. Phosphofructokinase, D. Haemoglobin
D: Haemoglobin
Answer: D: Haemoglobin 
Explanation: D i.e. Hemoglobin