(i) The reaction between carbonyl compounds and HCN proceeds via the nucleophilic attack of CN$^{-}$ on carbonyl carbon. Such an attack takes place faster if the carbonyl carbon is not attached to large alkyl groups. Less is the steric hindrance, morereactive is the compound. ∴ The order is :(ii) The presence of any electron withdrawing group or electronegative atom in carboxylic acid molecule increases the acidic strength of the acid. Moreover, the acidity decreases if the atom/group is bonded further off the α-carbon. Therefore, the acidstrength is : Between I and II the higher acidity of II is due to lesser number of $-\mathrm{CH}_3$ groups on the α-carbon. $-\mathrm{CH}_3$ group increases the electron density by +I effect on a-carbon which in turn exerts the same effect on COOH group and hinders release of $\mathrm{H}^+$ . (iii) In benzoic acid, the attachment of electron withdrawing groups (EWG) on the benzene ring makes the O—H bond of COOH group polar and H is lost as $\mathrm{H}^+$. Therefore, the order is,