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
The a-helix of proteins is: 
Select and explain. 
A. A pleated structure, B. Made periodic by disulphide bridges, C. A non-periodic structure, D. Stabilised by hydrogen bonds between NH and CO groups of the main chain
D: Stabilised by hydrogen bonds between NH and CO groups of the main chain
Answer: D: Stabilised by hydrogen bonds between NH and CO groups of the main chain 
Explanation: The alpha helix is the most common and stable conformation for a polypeptide chain. In proteins like hemoglobin and myoglobin, the alpha helix is abundant, whereas it is virtually absent in chymotrypsin. The structure is stabilized by hydrogen bonds between NH and C=O groups of the main chain.
Reference: Vasudevan 7th  ed, pg 37