The correct answer is B. Spin quantum number.
The spin quantum number is a quantum number that describes the intrinsic angular momentum of an electron. It is denoted by the symbol $m_s$ and can have two values, $+1/2$ and $-1/2$. The two electrons in an orbital can have different spin quantum numbers, which allows them to occupy the same orbital without violating the Pauli exclusion principle.
The principal quantum number, $n$, is the number that indicates the main energy level of an electron in an atom. It can have any integer value from 1 to infinity. The azimuthal quantum number, $l$, is the number that indicates the subshell of an electron in an atom. It can have values from 0 to $n-1$. The magnetic quantum number, $m_l$, is the number that indicates the orientation of an electron in an atom. It can have values from $-l$ to $l$, inclusive.
The following table shows the possible values of the quantum numbers for the first few energy levels:
Energy level $n$ | Principal quantum number $n$ | Azimuthal quantum number $l$ | Magnetic quantum number $m_l$
——- | ——– | ——– | ——–
1 | 1 | 0 | 0
2 | 2 | 0, 1 | -1, 0, +1
3 | 3 | 0, 1, 2 | -2, -1, 0, +1, +2
The Pauli exclusion principle states that no two electrons in an atom can have the same set of quantum numbers. This means that the two electrons in an orbital must have different spin quantum numbers.