In a multi-electron system, an orbital in a 4s subshell fills before an orbital in a 3d subshell despite having a higher quantum number $n$. Which of the following statements best explains this behavior?

A. A 3d orbital experiences less shielding than a 4s orbital.
B. The 3d subshell is able to hold more electrons than a 4s orbital, which subsequently results in a lower energy.
C. There are more nodes in a 3d orbital than a 4s orbital, causing an increase in energy.
D. A 4s orbital experiences more penetration than a 3d orbital.
E. The number of possible orientations for the 3d orbitals is higher than for 4s orbitals, resulting in a higher energy.

The correct option is D. A 4s orbital experiences more penetration than a 3d orbital. The correct explanation is that a 4s orbital experiences more penetration than a 3d orbital, which results in the 4s orbital filling first despite its higher principal quantum number.

The behaviour of electron filling in a multi-electron system, where a 4s subshell fills before a 3d subshell, can be explained by the concept of orbital penetration and shielding. Statement D correctly explains that a 4s orbital experiences more penetration than a 3d orbital. In essence, the 4s orbital can penetrate closer to the nucleus where the electron experiences a stronger nuclear attraction, thereby stabilizing it relative to the 3d orbital.

Though the 4s orbital has a higher principal quantum number, its electrons penetrate more effectively and experience less shielding from the inner electrons compared with those in the 3d orbital. Consequently, the 4s orbital is filled with electrons before the 3d. Hence the correct option is D.