ns→np→(n-1)d→(n-2)f
ns→(n-2)f→np→(n-1)d
ns→(n-2)f→(n-1)d→np
ns→(n-1)f→np→(n-2)d
If the principle quantum number n = 6, the correct sequence of filling of electron will be ns→(n-2)f→(n-1)d→np.
Here, n=6. Hence, the correct sequence of filling of electron will be 6s→4f→5d→6p.