Chọn đáp án A
Số thí nghiệm xảy ra phản ứng oxi hóa – khử là: (1), (2), (3), (4), (8).
$2\text{Mg}+{\text{O}}_2\to 2\text{MgO}$
$1,5{\text{Cl}}_2+3{\text{FeSO}}_4\to {\text{Fe}}_2({\text{SO}}_4{)}_3+{\text{FeCl}}_3$
$6\text{Fe}({\text{NO}}_3{)}_2+9{\text{H}}_2{\text{SO}}_4\to 3{\text{Fe}}_2({\text{SO}}_4{)}_3+10{\text{HNO}}_3+\text{NO}+4{\text{H}}_2\text{O}$
$3{\text{Br}}_2+8\text{NaOH}+2{\text{NaCrO}}_2\to 8{\text{H}}_2\text{O}+2{\text{Na}}_2{\text{CrO}}_4+6\text{NaBr}$
${\text{H}}_2\text{O}+2\text{KOH}+\text{Si}\to {\text{K}}_2{\text{SiO}}_3+2{\text{H}}_2$