Which of the following statement is correct?

According to Aufbau Principle,  once the 7s orbital is filled, then the next orbital which is to be filled is 4f, and only then do we move to the filled 6d orbital. But in electronic configuration actinium $\mathrm{Rn}7{\mathrm{s}}^{2}6{\mathrm{d}}^1$, it has been seen that after the 7s orbital, the 6d orbital gets filled giving it the electronic configuration. So,  the correct answer is B.