The correct option is A
Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
The Pauli exclusion principle is the quantum mechanical principle which states that two or more identical fermions (particles with half-integer spin) cannot occupy the same quantum state within a quantum system simultaneously.
In the case of electrons in atoms, it can be stated as follows: it is impossible for two electrons of a poly-electron atom to have the same values of the four quantum numbers: $$n$$, the principal quantum number, $$l$$, the angular momentum quantum number, $$m$$, the magnetic quantum number, and $$s$$, the spin quantum number.
For example, if two electrons reside in the same orbital, and if their $$n$$, $$l$$, and $$m$$ values are the same, then their $$s$$ must be different, and thus the electrons must have opposite half-integer spin projections of 1/2 and −1/2