All structures given below are of vitamin C. Most stable of them is:

In most keto-enol tautomeric systems, the keto form is more stable than the enol form because the $$C=O$$ bond is generally more stable than a $$C=C$$ bond accompanied by an $$-OH$$ group.

However, vitamin C (L-ascorbic acid) is an exception because its enediol form is highly stabilized by resonance.

In the given structure, the $$C=C$$ bond is conjugated with the carbonyl group of the lactone ring. This forms an $$\alpha,\beta$$-unsaturated lactone system in which the $$\pi$$ electrons are delocalized over multiple atoms.

This extended conjugation lowers the energy of the molecule and significantly increases its stability.

The two hydroxyl groups attached to the double bond constitute an enediol system. The lone pairs present on the oxygen atoms can participate in resonance with the conjugated ring system, providing additional stabilization.

Any tautomeric form that disrupts this conjugation would lose a substantial amount of resonance stabilization and would therefore be less stable.

Thus, the enediol structure containing the conjugated $$\alpha,\beta$$-unsaturated lactone system is the most stable form of vitamin C.

Hence, the correct answer is the enediol form shown in Option A.