The atomic numbers of elements x, y and z are 19, 21 and 25 respectively. The number of electrons present in M-shell of these elements follow the order                                             

$x=19\Rightarrow$The electronic configuration is $1s^{2}2s^{2}2p^6\underset{Mshell}{\underbrace{3s^{2}3p^6}}4s^1.$

$y=21\Rightarrow$The electronic  configuration is $1s^{1}2s^{2}2p^6\underset{Mshell}{\underbrace{3s^{2}3p^{6}3d^1}}4s^2.$

$z=25\Rightarrow$The electronic configuration is $1s^{1}2s^{2}2p^6\underset{Mshell}{\underbrace{3s^{2}3p^{6}3d^5}}4s^2$

$\therefore$The number of electrons present in M shell of these element follow the order $z>y>x$