If n = 6, the correct sequence for filling of electrons will be

ns < (n-2)f < (n-2)d < np is the correct increasing order of energy. We know l = 0,1, 2, 3 for s, p, d, f subshells(n - 2)f, (n - 1)d and np orbitals have same (n + l) value but 'n' value in (n - 2)f < (n - 1)d < npEx:- 4f < 5d < 6p