Carbonyl compounds undergo a variety of nucleophilic reactions....

The formation of cyclohexanone cyanohydrin involves the reaction of cyclohexanone with hydrogen cyanide (HCN). The cyanide ion ($CN^-$) acts as a nucleophile and attacks the electrophilic carbonyl carbon, forming a tetrahedral alkoxide intermediate. This intermediate is then protonated to yield the cyanohydrin. The entire process is a pure nucleophilic addition reaction, and no small molecule like water is eliminated.

In contrast, the formation of cyclohexanone oxime (from hydroxylamine), cyclohexanone semicarbazone (from semicarbazide), and cyclohexanone ethylene ketal (from ethylene glycol) all proceed via an initial nucleophilic addition followed by the elimination of a water molecule (nucleophilic addition-elimination mechanism).

Answer: Cyclohexanone cyanohydrin