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

1. a

   (i) $\ce{Cr2O3}$, $770\text{ K}$, $20\text{ atm}$; (ii) $\ce{CrO2Cl2}$, $\ce{H3O+}$; (iii) $\ce{NaOH}$; (iv) $\ce{H3O+}$

   ✓
2. b

   (i) $\ce{CrO2Cl2}$, $\ce{H3O+}$; (ii) $\ce{Cr2O3}$, $770\text{ K}$, $20\text{ atm}$; (iii) $\ce{NaOH}$; (iv) $\ce{H3O+}$

3. c

   (i) $\ce{KMnO4}$, $\ce{OH-}$; (ii) $\ce{Mo2O3}$, $\Delta$; (iii) $\ce{NaOH}$; (iv) $\ce{H3O+}$

4. d

   (i) $\ce{Mo2O3}$, $\Delta$; (ii) $\ce{CrO2Cl2H3O+}$; (iii) $\ce{NaOH}$; (iv) $\ce{H3O+}$

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