Bài giải:

$\left( \frac{1}{2}xy+{{x}^{3}}{{y}^{3}}+{{y}^{5}}z+\frac{2}{5}+{{x}^{5}} \right)+\left( \frac{3}{4}xy+2{{x}^{3}}{{y}^{3}}-4{{y}^{5}}z+\frac{3}{5}+{{x}^{5}} \right)$

$=\left( \frac{1}{2}xy+\frac{3}{4}xy \right)+\left( {{x}^{3}}{{y}^{3}}+2{{x}^{3}}{{y}^{3}} \right)+\left( {{y}^{5}}z-4{{y}^{5}}z \right)+\left( \frac{2}{5}+\frac{3}{5} \right)+\left( {{x}^{5}}+{{x}^{5}} \right)$

=$\frac{5}{4}xy+3{{x}^{3}}{{y}^{3}}-3{{y}^{5}}z+1+2{{x}^{5}}$

Đáp án đúng là C