Which among the following compounds has the highest boiling point?

If $y_k$ is the $k$-th derivative of $y$ with respect to $x$, and $y = \cos(\sin x)$, then $y_1 \sin x + y_2 \cos x$ is equal to

If $x=a\left[\cos \theta+\log \left\{\tan \left(\frac{\theta}{2}\right)\right\}\right]$ and $y=a \sin \theta$,then $\frac{dy}{dx}$ is equal to

Energy needed in the breaking of a drop of radius $R$ into $n$ drops of radius $r$ is given by : (With $S$ surface tension and $P$ atmospheric pressure)

$A$ feasible solution to an $LP$ problem,

Starting from rest,a body slides down a $45^{\circ}$ inclined plane in twice the time it takes to slide down the same distance in the absence of friction. The coefficient of friction between the body and the inclined plane is: