Monosaccharides have —-CHO (or C = O) and —OH groups, so they undergo usual oxidation and reduction. Further, monosaccharides form osazone when treated with excess of phenylhydrazine (3 equivalents). In osazone formation only the first two carbon atoms are involved. Thus monosaccharides having identical configuration on rest of C atoms except first two will form same osazone, as is the case with glucose and fructose. A, Band C are three hexoses and form same osazone D. Compounds A to D behave as below : 
(i) $\mathrm{D}\stackrel{\mathrm{HCl}}{⟶}\underset{{\mathrm{CH}}_3\mathrm{COOH}}{\stackrel{\mathrm{Zn}}{⟶}}\mathrm{D}-\text{ Fructose }$
(ii) $A\stackrel{Ni,H_2}{⟶}\stackrel{{HNO}_3}{⟶}\underset{H_3{O}^{+}}{\stackrel{Na-Hg}{⟶}}B+C$
(iii) $\mathrm{B}\stackrel{{\mathrm{HNO}}_3}{⟶}\text{ Optically active glycaric acid }$
(iv) $\mathrm{C}\stackrel{{\mathrm{HNO}}_3}{⟶}\text{ Optically inactive glycaric acid }$