Đáp án đúng: A FeCl3. $\begin{array}{l}\text{F}{{\text{e}}_{\text{2}}}{{\text{O}}_{\text{3}}}\text{+3}{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\xrightarrow{{}}\text{F}{{\text{e}}_{\text{2}}}{{\text{(S}{{\text{O}}_{\text{4}}}\text{)}}_{\text{3}}}\text{+3}{{\text{H}}_{\text{2}}}\text{O}\\\text{F}{{\text{e}}_{\text{3}}}{{\text{O}}_{\text{4}}}\text{+4}{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\xrightarrow{{}}\text{FeS}{{\text{O}}_{\text{4}}}\text{+F}{{\text{e}}_{\text{2}}}{{\text{(S}{{\text{O}}_{\text{4}}}\text{)}}_{\text{3}}}\text{+4}{{\text{H}}_{\text{2}}}\text{O}\\\text{2Fe(OH}{{\text{)}}_{\text{3}}}\text{+3}{{\text{H}}_{\text{2}}}\text{S}{{\text{O}}_{\text{4}}}\xrightarrow{{}}\text{F}{{\text{e}}_{\text{2}}}{{\text{(S}{{\text{O}}_{\text{4}}}\text{)}}_{\text{3}}}\text{+3}{{\text{H}}_{\text{2}}}\text{O}\end{array}$
