The correct option is C Aufbau principle
The electronic configuration of Cr is 1s22s22p63s23p63d54s1. According to Aufbau principle, hypothetically, electrons orbiting one or more atoms fill the lowest available energy levels before filling higher levels (e.g., 1s before 2s). In this way, the electrons of an atom, molecule, or ion harmonize into the most stable electron configuration possible. Therefore, according to this principle, the configuration should be 1s22s22p63s23p63d44s2. However, due to the greater stability of half-filled configuration, the configuration of chromium is against the Aufbau principle.