The correct option for a redox couple is:

On balancing the given redox reaction,

$$\mathrm{aCr}_{2} \mathrm{O}_{7}^{2-}+\mathrm{bSO}_{3}^{2-}(\mathrm{aq})+\mathrm{cH}^{+}(\mathrm{aq}) \rightarrow 2 \mathrm{aCr}^{3+}(\mathrm{aq})+\mathrm{bSO}_{4}^{2-}(\mathrm{aq})+\frac{\mathrm{c}}{2} \mathrm{H}_{2} \mathrm{O}(\ell)$$

the coefficients $$\mathrm{a}, \mathrm{b}$$ and $$\mathrm{c}$$ are found to be, respectively -

Which of the following reactions is a decomposition redox reaction?