Consider the following statement is $S$ and $R$ ;

$S$. Both $\sin x$ and $\cos x$ are decreasing function in the interval $\left(\dfrac {\pi}{2}, \pi \right)$

$R:$ If a differentiable function decreases in an interval $(a, b)$ then its derivative also decreases in $(a, b)$, which of the following is true?

From the graph, it is clear that both $\sin x$ and $\cos x$ in the internal $(\pi /2, \pi)$ are the decreasing functions.

Therefore, $S$ is correct.

To disprove $R$ let us consider the counter example,

$f(x)=\sin x$ in $(0, \pi/2)$

so that $f'(x)=\cos x$

again from the graph, it is clear that $f(x)$ is increasing in $(0, \pi /2)$, but $f'(x)$ is decreasing in $(0, \pi /2)$

Therefore, $R$ is wrong. Therefore, d is the correct option.