Is the Aufbau principle violated in the diagram (b)? In which of the following diagrams is the Aufbau principle violated?


Ignoring the Hund principle, the Aufbau principle states that electrons have to first fill the lowest energy levels. Now the energy level of the s-orbital has to be the lowest, and only in the option (b) the s-orbital is not filled fully. So the answer is (b).
Is my reasoning correct?
Since the question specifically asks about the violation of Aufbau principle, your reasoning is correct.  

Had it been based on the violation of Hund's rule then option (a) would be the answer.