Bài giải:

$\left( \frac{1}{2}x{{y}^{2}}+{{x}^{2}}-{{x}^{2}}y \right)-A=-x{{y}^{2}}+{{x}^{2}}y+2$

A = $\left( \frac{1}{2}x{{y}^{2}}+{{x}^{2}}-{{x}^{2}}y \right)-\left( -x{{y}^{2}}+{{x}^{2}}y+2 \right)$

A = $\frac{1}{2}x{{y}^{2}}+{{x}^{2}}-{{x}^{2}}y+x{{y}^{2}}-{{x}^{2}}y-2$

A = $\left( \frac{1}{2}x{{y}^{2}}+x{{y}^{2}} \right)+{{x}^{2}}+(-{{x}^{2}}y-{{x}^{2}}y)-2$

A = $\frac{3}{2}x{{y}^{2}}+{{x}^{2}}-2{{x}^{2}}y-2$

Đáp án đúng là: B