The formulas of A and B for the following reaction sequence are

Fructose is a polyhydroxy ketone, It reacts with HCN to form a cyanohydrin which on hydrolysis forms a alpha hydroxy carboxylic group.

Fructose on reaction ${NaBH}_{4}$ forms D-sorbitol and D-mannitol( Mannitol and sorbitol are isomers, the only difference being the orientation of the hydroxyl group on carbon 2), which on reduction with HI in presence of red phosphorous form n-hexane.