Which of the following functions is decreasing on (0, (π)/2)?

The correct answer is Option (3) → $\cos x$ ##

In the interval $(0, \frac{\pi}{2})$, $f(x) = \cos x$

$\Rightarrow f'(x) = -\sin x$

which gives $f'(x) < 0$ in $(0, \frac{\pi}{2})$.

Hence, $f(x) = \cos x$ is decreasing in $(0, \frac{\pi}{2})$.