Which from following rule / principle states that "No two electrons in an atom can have the same set of four quantum numbers"?

Calculate the wavenumber of the photon emitted during transition from the orbit of $$n=2$$ to $$n=1$$ in hydrogen atom. $$\left[R_H=109677 \mathrm{~cm}^{-1}\right]$$

What is maximum number of electrons accommodated in a subshell having azimuthal quantum number, $$\ell$$ = 2 ?

How many values of magnetic quantum number are possible for each value of azimuthal quantum number?