Which structure of protein remains intact after coagulation of egg...

When an egg is boiled, the soluble globular protein, albumin present in it is converted to insoluble fibrous protein. 

During denaturation, its biological activity is lost. Boiling an egg causes denaturation of its protein resulting in loss of its quaternary, tertiary and secondary structures.

But the primary structure (representing the sequence of $\alpha$ amino acids) remains intact.

Hence option A is the answer.