1. Key principle: precipitation by $H_2S$ depends on $[S^{2-}]$ in the medium

   In aqueous solution, $H_2S$ is a weak acid: $H_2S \rightleftharpoons H^+ + HS^-$ $HS^- \rightleftharpoons H^+ + S^{2-}$

   In an acidified solution, the concentration of $H^+$ is high, so both equilibria are suppressed to the left. Hence, the concentration of $S^{2-}$ remains very small.

2. Which metal sulfides precipitate in acidic medium?

   A metal sulfide $MS$ will precipitate if its ionic product exceeds its solubility product: $[M^{2+}][S^{2-}] > K_{sp}$

   Since $[S^{2-}]$ is very low in acidic medium, only those metal ions whose sulfides have extremely low $K_{sp}$ will precipitate.

3. Behavior of the given ions

   • $Cu^{2+}$ forms $CuS$, which is very insoluble and precipitates even in acidic medium.
   • $Hg^{2+}$ forms $HgS$, which is extremely insoluble and also precipitates in acidic medium.
   • $Ni^{2+}$ forms $NiS$, but it generally precipitates in alkaline/ammoniacal medium where $[S^{2-}]$ is higher.
   • $Mn^{2+}$ forms $MnS$, which also requires basic medium for precipitation.

4. Conclusion

   Therefore, on passing $H_2S$ through an acidified solution containing $Mn^{2+}, Ni^{2+}, Cu^{2+}, Hg^{2+}$, the ions precipitated are: $CuS \text{ and } HgS$

5. Option check

   • A: $CuS$ and $HgS$ ✅
   • B: $MnS$ and $CuS$ ❌ ($MnS$ does not precipitate in acidic medium)
   • C: $MnS$ and $NiS$ ❌
   • D: $NiS$ and $HgS$ ❌ ($NiS$ does not precipitate in acidic medium)

Hence, the correct answer is A.