In the given picture ,it represents maturation of pro-insulin into insulin ,identify 'A' in the picture ?

C-peptide .

Insulin is synthesised as a pro-hormone (like a pro-enzyme, the pro-hormone also needs to be processed before it becomes a fully mature and functional hormone) which contains an extra stretch called the C peptide. This C peptide is not present in the mature insulin and is removed during maturation into insulin.