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

According to the $(n+l)$ rule and the Aufbau principle, for $n=6$:$6s$: $n+l=6+0=6$$4f$: $n+l=4+3=7$$5d$: $n+l=5+2=7$ (filled after $4f$ as $n$ is higher)$6p$: $n+l=6+1=7$ (filled after $5d$ as $n$ is higher)Sequence: $6s\to 4f\to 5d\to 6p$, which corresponds to $ns\to (n-2)f\to (n-1)d\to np$.