Which of the following electronic configurations has the least energy?

Given below are the quantum numbers for $4$ electrons.
$A. \; n = 3, l = 2, m_l = 1, m_s = +1/2$
$B. \; n = 4, l = 1, m_l = 0, m_s = +1/2$
$C. \; n = 4, l = 2, m_l = -2, m_s = -1/2$
$D. \; n = 3, l = 1, m_l = -1, m_s = +1/2$
The correct order of increasing energy is

The total number of magnetic quantum numbers for a $d$-orbital is given by

$A$ completely filled $d$ orbital $(d^{10})$ configuration is:

The principal quantum number represents:

Explain the Aufbau principle with an example.