The correct sequence of reagents for the preparation of $Q$ and $R$ is:

• Correct answer
  A

  (i) ${\mathrm{Cr}}_2{\mathrm{O}}_3, 770 \mathrm{K}, 20 \mathrm{atm}$; (ii) ${\mathrm{CrO}}_2{\mathrm{Cl}}_2, {\mathrm{H}}_3{\mathrm{O}}^{+}$; (iii) $\mathrm{NaOH}$; (iv) ${\mathrm{H}}_3{\mathrm{O}}^{+}$
• B

  (i) ${\mathrm{CrO}}_2{\mathrm{Cl}}_2, {\mathrm{H}}_3{\mathrm{O}}^{+}$; (ii) ${\mathrm{Cr}}_2{\mathrm{O}}_3, 770 \mathrm{K}, 20 \mathrm{atm}$; (iii) $\mathrm{NaOH}$; (iv) ${\mathrm{H}}_3{\mathrm{O}}^{+}$
• C

  (i) ${\mathrm{KMnO}}_4, {\mathrm{OH}}^{-}$; (ii) ${\mathrm{Mo}}_2{\mathrm{O}}_3, \mathrm{\Delta }$; (iii) $\mathrm{NaOH}$; (iv) ${\mathrm{H}}_3{\mathrm{O}}^{+}$
• D

  (i) ${\mathrm{Mo}}_2{\mathrm{O}}_3, \mathrm{\Delta }$; (ii) ${\mathrm{CrO}}_2{\mathrm{Cl}}_2, {\mathrm{H}}_3{\mathrm{O}}^{+}$; (iii) $\mathrm{NaOH}$; (iv) ${\mathrm{H}}_3{\mathrm{O}}^{+}$

Hence, the correct answer is option: 'A'.