Bài giải:

$\frac{x-y}{{{x}^{3}}+{{y}^{3}}}.P=\frac{{{x}^{2}}-2xy+{{y}^{2}}}{{{x}^{2}}-xy+{{y}^{2}}}$

$P=\frac{{{x}^{2}}-2xy+{{y}^{2}}}{{{x}^{2}}-xy+{{y}^{2}}}:\frac{x-y}{{{x}^{3}}+{{y}^{3}}}$

$P=\frac{{{\left( x-y \right)}^{2}}}{{{x}^{2}}-xy+{{y}^{2}}}.\frac{\left( x+y \right)\left( {{x}^{2}}-xy+{{y}^{2}} \right)}{x-y}$

$P=\left( x-y \right)\left( x+y \right)={{x}^{2}}-{{y}^{2}}$

Đáp án đúng là A