If the principal quantum number is n = 6, the correct sequence of filling of electrons will be :

If the fundamental quantum number is 6, the proper order for filling one electron will be $\mathrm{ns}\to (\mathrm{n}-2)\mathrm{f}\to (\mathrm{n}-1)\mathrm{d}\to \mathrm{np}$
Here, n=6. Therefore, the proper order of electron filling will be $6\mathrm{s}\to 4\mathrm{f}\to 5\mathrm{d}\to 6\mathrm{p}.$