What is the total number of electrons present in completely filled...

For the 3s subshell, $l = 0$ and $m_l = 0$. It contains 1 orbital with 2 electrons.

For the 3p subshell, $l = 1$ and $m_l = -1, 0, +1$. It contains 3 orbitals, each with 2 electrons. The number of electrons with $m_l = 0$ is 2.

For the 3d subshell, $l = 2$ and $m_l = -2, -1, 0, +1, +2$. It contains 5 orbitals, each with 2 electrons. The number of electrons with $m_l = 0$ is 2.

The only magnetic quantum number common to all three subshells is $m_l = 0$.

Total number of electrons having the same magnetic quantum number (m_l = 0$) across these completely filled subshells is 2 + 2 + 2 = 6$.

Answer: $6$