The atomic number of an element 'M' is 26. How many electrons are present in the M-shell of the element in its M³⁺ state ?

An atomic number of element M is 26.
Electronic configuration is 1s² 2s ² 2p⁶ 3s ² 3p⁶ 4s ² 3d⁶

first 2 electrons are removed form 4s shell then from 3d shell.

M⁺³  electronic configuration will be 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵
 

K-shell is n=1
L shell is n=2
M shell is =3.
For M shell, no. of electrons present will be 2+6+5=13 electrons.