(d)
Let $I = \int_{0}^{\pi/2}\frac{\sqrt{\cot x}}{\sqrt{\cot x} + \sqrt{\tan x}}dx$ …(i)
$\Rightarrow \ I = \int_{0}^{\pi/2}{\frac{\sqrt{\tan x}}{\sqrt{\cot x} + \sqrt{\tan x}}dx}$ …(ii)
$\left\lbrack put\ x = \left( \frac{\pi}{2} - x \right) \right\rbrack$
On adding Eqs. (i) and (ii), we get
$2I = \int_{0}^{\pi/2}{1\ dx = \frac{\pi}{2}\ \Rightarrow \ I = \frac{\pi}{4}}$