Đáp án: $\textbf{B}$

Giải thích các bước giải:

$\left(1 - 2\sqrt{\dfrac{a}{b}} + \dfrac{a}{b}\right) : \left(a^{\frac{1}{2}} - b^{\frac{1}{2}}\right)^2$ 

$= \left[\left(\sqrt{\dfrac{a}{b}}\right)^2 - 2\sqrt{\dfrac{a}{b}} + 1\right) : \left(\sqrt{a} - \sqrt{b}\right)^2$

$= \left(\sqrt{\dfrac{a}{b}} - 1\right)^2 : \left(\sqrt{a} - \sqrt{b}\right)^2$

$= \left(\dfrac{\sqrt{a} - \sqrt{b}}{\sqrt{b}}\right)^2 : \left(\sqrt{a} - \sqrt{b}\right)^2$

$= \dfrac{1}{b}\left(\sqrt{a} - \sqrt{b}\right)^2 :\left(\sqrt{a} - \sqrt{b}\right)^2$

$= \dfrac{1}{b}$

$\Rightarrow \textbf{B}$