Electron configuration of phosphorus reveals occupied orbitals Steps:

• Phosphorus has atomic number 15, requiring 15 electrons in its ground state.
• Apply Aufbau principle to fill orbitals: 1s² 2s² 2p⁶ 3s² 3p³.
• Occupied atomic orbitals are 1s, 2s, 2p, 3s, 3p.
• Choice C identifies the occupied s subshells in the second and third principal energy levels.

Why C is correct:

• Phosphorus configuration [Ne] 3s² 3p³ includes filled 2s² (from neon core) and 3s², per Aufbau filling order.

Why the others are wrong:

• A: 1s is occupied, but 2p notation ignores 3s/3p valence; incomplete for phosphorus.
• B: 2d does not exist (d orbitals start at n=3) and is unoccupied in phosphorus.
• D: 3d is unoccupied (higher energy, empty until transition metals).

Final answer: C