Which of the following is an incorrect pair?

Which of the following is a hormone-releasing $IUD$?

Which of the following represents soap?

In the photoelectric effect,if the work functions of $Li$,$Na$,and $Rb$ are $2.41 \ eV$,$2.30 \ eV$,and $2.09 \ eV$ respectively,what could be the approximate work function of $K$ in $eV$?

$A$ weightless thread can bear a tension up to $3.7\,kg-wt$. $A$ stone of mass $500\,g$ is tied to it and revolved in a circular path of radius $4\,m$ in a vertical plane. If $g = 10\,ms^{-2}$,then the maximum angular velocity of the stone will be ......... $rad\,s^{-1}$