Bài giải:

$x{{\left( x+1 \right)}^{4}}+x{{\left( x+1 \right)}^{3}}+x{{\left( x+1 \right)}^{2}}+{{\left( x+1 \right)}^{2}}$

=$x{{\left( x+1 \right)}^{4}}+x{{\left( x+1 \right)}^{3}}+\left[ x{{\left( x+1 \right)}^{2}}+{{\left( x+1 \right)}^{2}} \right]$

=$x{{\left( x+1 \right)}^{4}}+x{{\left( x+1 \right)}^{3}}+{{\left( x+1 \right)}^{2}}.\left( x+1 \right)$

=$x{{\left( x+1 \right)}^{4}}+x{{\left( x+1 \right)}^{3}}+{{\left( x+1 \right)}^{3}}$

=$x{{\left( x+1 \right)}^{4}}+{{\left( x+1 \right)}^{3}}\left( x+1 \right)$

=$x{{\left( x+1 \right)}^{4}}+{{\left( x+1 \right)}^{4}}$

=${{\left( x+1 \right)}^{4}}\left( x+1 \right)$

=${{\left( x+1 \right)}^{5}}$

Đáp án đúng là B