When aldehydes are reduced using NaBH₄, the key intermediate formed is an alkoxide ion. The hydride ion (H^-) from NaBH₄ attacks the electrophilic carbonyl carbon of the aldehyde, forming an alkoxide intermediate. This intermediate is then protonated to yield the corresponding alcohol.

The other options are incorrect:

Carbanions are not involved as the mechanism does not require electron pair donation to carbon.

Carbocations are not formed because the reaction conditions do not favor the generation of positively charged carbon species.

Carbenes are not relevant as they do not participate in hydride transfer reactions.

A carbanion is a negatively charged carbon atom with extra electrons. In the reduction of aldehydes with NaBH₄, a carbanion is not formed because the reaction mechanism involves hydride ion transfer rather than electron pair shifts that form carbanions.

A carbocation is a positively charged carbon atom with an incomplete octet. The reduction of aldehydes with NaBH₄ does not involve the formation of carbocations, as the BH₄^- ion acts as a hydride donor, not an acid that would generate such species.

A carbene is a neutral species with a divalent carbon atom. Carbenes are not formed during the reduction of aldehydes with NaBH₄, as the reaction involves hydride transfer rather than the creation of divalent carbon intermediates.

The BH₄^- ion from NaBH₄ donates a hydride ion to the carbonyl carbon of the aldehyde, resulting in the formation of an alkoxide intermediate. This species is characterized by a negatively charged oxygen that is later protonated to form the corresponding alcohol.