What is the hybridization of the carbonyl carbon?

sp²

The correct answer is option 2. \(sp^2\)

The carbonyl carbon atom is \(sp^2\)-hybridized and forms three sigma \((\sigma )\) bonds. The fourth valence electron of carbon remains in its p-orbital and forms a \(\pi \)-bond with oxygen by overlap with p-orbital of an oxygen. the oxygen atom also has two non bonding electron pairs. Thus, the carbonyl carbon and the three atoms attached to it lie in the same plane and the π-electron cloud is above and below this plane. The bond angles are approximately 120° as expected of a trigonal coplanar structure.