Hướng dẫn:

${{n}_{{{O}_{2}}}}=\frac{{{V}_{{{O}_{2}}}}}{22,4}=\frac{5,6}{22,4}=0,25\left( mol \right)$

  $4P+5{{O}_{2}}\xrightarrow{{{t}^{0}}}2{{P}_{2}}{{O}_{5}}$

 0,2$\leftarrow $0,25                       (mol)

Theo phương trình ta có: ${{n}_{P}}=\frac{4.{{n}_{{{O}_{2}}}}}{5}=\frac{4.0,25}{5}=0,2\left( mol \right)$

$\Rightarrow {{m}_{P}}={{n}_{P}}.{{M}_{P}}=0,2.31=6,2\left( g \right)$

$\Rightarrow $ Chọn đáp án A