1. Identify the compound

   Ferrous ammonium sulphate is Mohr’s salt, with formula $\mathrm{FeSO_4\cdot (NH_4)_2SO_4\cdot 6H_2O}$ It contains iron in the +2 oxidation state.

2. What happens on prolonged heating?

   Ferrous ion, $\mathrm{Fe^{2+}}$, is easily oxidised to ferric ion, $\mathrm{Fe^{3+}}$, especially during heating in air.

   $\mathrm{Fe^{2+} \rightarrow Fe^{3+}}$

   Since Mohr’s salt is prepared to retain iron in the ferrous state, prolonged heating is avoided.

3. Check the options

   • A: prevent hydrolysis — not the main reason here.
   • B: prevent breaking — irrelevant.
   • C: prevent oxidation — correct, because $\mathrm{Fe^{2+}}$ gets oxidised to $\mathrm{Fe^{3+}}$.
   • D: prevent reduction — incorrect.

4. Conclusion

   Prolonged heating is avoided during the preparation of ferrous ammonium sulphate to prevent oxidation of $\mathrm{Fe^{2+}}$.

Therefore, the correct option is C.