If $n = 3$,then which of the following values of $l$ is incorrect?

While filling up of electrons in the atomic orbitals,the $4s$ orbital is filled before the $3d$ orbital but reverse happens during the ionisation of the atom. Explain why?

Figure $1$ shows the electron density distribution for a $2s$ orbital. Figure $2$ shows the radial wave function plot for a $2s$ orbital. Which of the following points $(A, B, C, D)$ in Figure $2$ most accurately represents the spherical nodal surface as shown in Figure $1$?

Which of the following sets of quantum numbers represents the $19^{\text{th}}$ electron of $Cr$ $(Z=24)$?

Explain,giving reasons,which of the following sets of quantum numbers are not possible.
$(a)$ $n=0, l=0, m_{l}=0, m_{s}=+\frac{1}{2}$
$(b)$ $n=1, l=0, m_{l}=0, m_{s}=-\frac{1}{2}$
$(c)$ $n=1, l=1, m_{l}=0, m_{s}=+\frac{1}{2}$
$(d)$ $n=2, l=1, m_{l}=0, m_{s}=-\frac{1}{2}$
$(e)$ $n=3, l=3, m_{l}=-3, m_{s}=+\frac{1}{2}$
$(f)$ $n=3, l=1, m_{l}=0, m_{s}=+\frac{1}{2}$

Statement $A$: An orbital can accommodate a maximum of two electrons.
Reason $R$: Two electrons in an orbital produce opposite magnetic fields.