The process of addition of water to an ethyne in the presence of H₂SO₄ and HgSO₄ is known as hydration of alkynes.

When hydration of butyne takes place, the product is a ketone.

The OH group gets attached to the secondary carbocation (which is more stable due to the electron giving the property of alkyl group attached to it) after breaking of the triple bond.

The formation of an intermediate enol takes place. The deprotonation of oxygen takes place (i.e. hydrogen of oxygen is removed) to give a more stable final product which is ketone.

This process is called tautomerism .

Thus, the structure of A is pro-2-ene-1-ol and isomerization is tautomerism.