Bài giải:

$A-B=\left( 2{{x}^{3}}-4{{x}^{2}}y+1\frac{1}{3}x{{y}^{2}}-{{y}^{4}}+1 \right)-\left( -2{{x}^{3}}-1\frac{1}{2}{{x}^{2}}y+\frac{2}{3}x{{y}^{2}}-{{y}^{4}}-3 \right)$

=$2{{x}^{3}}-4{{x}^{2}}y+1\frac{1}{3}x{{y}^{2}}-{{y}^{4}}+1+2{{x}^{3}}+1\frac{1}{2}{{x}^{2}}y-\frac{2}{3}x{{y}^{2}}+{{y}^{4}}+3$

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

=$4{{x}^{3}}-2\frac{1}{2}{{x}^{2}}y+\frac{2}{3}x{{y}^{2}}+4$

Đáp án đúng là B