Correct Answer - 4
Hydrogen sulphide acts as a strong reducing agent as it decomposes evolving hydrogen .Acidified `KMnO_(4)` is decolourised:
`{:(2KMnO_(4)+3H_(2)SO_(4) rarr K_(2)SO_(4)+2MnSO_(4)+3H_(2)O+5[O]),([H_(2)S+O rarr H_(2)O+S]xx5),(bar(2KMnO_(4)+3H_(2)SO_(4)+5H_(2)S rarr K_(2)SO_(4)+2MnSO_(4) +8H_(2)O+5S)):}`