Correct: B
The electronic configuration of Fe (Z=26) is [Ar] 3d^6 4s^2. However, the electrons can be arranged in different ways, giving rise to different configurations. Using the Aufbau principle and the Pauli exclusion principle, the possible configurations are [Ar] 3d^6 4s^2, [Ar] 3d^7 4s^1, and [Ar] 3d^8. However, only [Ar] 3d^6 4s^2 and [Ar] 3d^7 4s^1 are stable