- Chọn A.

- Theo ĐLBTKL khối lượng oxi tham gia phản ứng là: ${{m}_{{{O}_{2}}}}=4\left( gam \right)\to {{n}_{{{O}_{2}}}}=\frac{4}{16}=0,25\left( mol \right)$

2Mg      +   ${{O}_{2}}$ $\xrightarrow{{{t}^{o}}}$  2MgO                2Al       +     3 ${{O}_{2}}$$\xrightarrow{{{t}^{o}}}$   2$A{{l}_{2}}{{O}_{3}}$

  2$\left( mol \right)$        1$\left( mol \right)$                                 2$\left( mol \right)$         3$\left( mol \right)$

     x $\left( mol \right)$ $\to $ $\frac{x}{2}$$\left( mol \right)$$\left( 1 \right)$                        x$\left( mol \right)$  $\to $$\frac{3x}{2}$$\left( mol \right)$$\left( 2 \right)$