Which of the following give an appreciable yield of a cyanohydrin on treatment with HCN in the presence of a catalytic amount of base?

Unhindered aldehydes and aliphatic or cyclic ketones add $\mathrm{CN}{}^{-}$ with a favourable equilibrium constant. Benzophenone is both sterically hindered and stabilised by conjugation with two phenyl rings, so the cyanohydrin equilibrium lies on the side of the ketone.