Bài giải:

$\frac{{{x}^{2}}+2ax+{{a}^{2}}}{{{x}^{2}}+2bx+{{b}^{2}}}.\frac{{{x}^{2}}+bx-\text{ax-ab}}{{{x}^{2}}-bx-ab+\text{ax}}$

=$\frac{{{\left( x+a \right)}^{2}}}{{{\left( x+b \right)}^{2}}}.\frac{x\left( x+b \right)-a\left( x+b \right)}{x\left( x-b \right)+a\left( x-b \right)}$

=$\frac{{{\left( x+a \right)}^{2}}}{{{\left( x+b \right)}^{2}}}.\frac{\left( x+b \right)\left( x-a \right)}{\left( x-b \right)\left( x+a \right)}$

=$\frac{x+a}{x+b}.\frac{x-a}{x-b}$

=$\frac{{{x}^{2}}-{{a}^{2}}}{{{x}^{2}}-{{b}^{2}}}$

Đáp án đúng là B