Đáp án đúng: B
4M.
$\begin{array}{l}\text{A}{{\text{l}}^{\text{3+}}}\,\,\text{+}\,\,\text{3O}{{\text{H}}^{\text{-}}}\,\xrightarrow{{}}\,\text{Al(OH}{{\text{)}}_{\text{3}}}\\\text{0,25}\,\,\to \,\text{0,75}\,\,\xrightarrow{{}}\,\,\,\,\,\text{0,25}\\\,\,\,\text{Al(OH}{{\text{)}}_{\text{3}}}\,\,\text{+}\,\,\,\text{O}{{\text{H}}^{\text{-}}}\,\xrightarrow{{}}\,\text{Al(OH)}_{\text{4}}^{\text{-}}\\\text{(0,25-0,2)}\,\to \,\,\text{0,05}\\\Rightarrow {{\text{n}}_{\text{O}{{\text{H}}^{\text{-}}}}}\text{=0,8}\,\Rightarrow \,{{\text{C}}_{\text{M}}}\,\text{=}\,\text{4M}\end{array}$
