According to the Aufbau Principle, orbitals are filled in the order of increasing energy, which is determined by the $(n+l)$ rule. For the 6th period ($n=6$), the orbitals involved are $6s, 4f, 5d,$ and $6p$. We calculate their $(n+l)$ values:

1. ns (6s): $n=6, l=0 \Rightarrow n+l = 6$
2. (n-2)f (4f): $n=4, l=3 \Rightarrow n+l = 7$
3. (n-1)d (5d): $n=5, l=2 \Rightarrow n+l = 7$
4. np (6p): $n=6, l=1 \Rightarrow n+l = 7$

When $(n+l)$ values are equal (as with $4f, 5d, 6p$), the orbital with the lower $n$ value has lower energy and is filled first. Therefore, the order is $4f < 5d < 6p$.

Combining with $6s$, the correct filling sequence is: $6s \to 4f \to 5d \to 6p$, which corresponds to the general form: $ns \to (n-2)f \to (n-1)d \to np$.