'A' in the given reaction is

The reaction takes place under acidic conditions and involves the condensation of pivalaldehyde with an $$\alpha$$-hydroxy acid (2-hydroxyisobutyric acid).

The carbonyl oxygen of the aldehyde is first protonated by $$H^+$$, increasing the electrophilicity of the carbonyl carbon.

The hydroxyl group $$(-OH)$$ of the $$\alpha$$-hydroxy acid attacks the protonated carbonyl carbon, forming a hemiacetal intermediate.

Subsequently, the carboxyl group participates in intramolecular cyclization. The oxygen of the $$-COOH$$ group attacks the hemiacetal carbon, followed by elimination of water, producing a stable five-membered cyclic product.

The resulting product is a cyclic dioxolanone, in which:

• One oxygen atom originates from the hydroxyl group.

• The second oxygen atom originates from the carboxyl group.

• The carbonyl $$\left(C=O\right)$$ of the original carboxylic acid is retained within the ring.

Among the given structures:

• Option A represents a cyclic acetal and lacks the required carbonyl group.

• Option B represents the correct five-membered cyclic dioxolanone.

• Option C is an open-chain ester and is not the major product under these conditions.

• Option D does not correspond to the expected cyclized product.

Hence, the major product formed is the cyclic dioxolanone shown in

$$\boxed{\text{Option B}}$$