In which of the following molecules, the central atom involves the expansion of octet?

Valence shell configuration of P is 3s² 3p³. It forms three bonds with three Cl atoms. This implies that no expansion of orbital of P is observed in PCl₃.

N has no empty orbital, hence N cannot expand its octet.

Valence shell configuration of Cl is 3s² 3p⁵ and to form three bonds with F atoms, Cl goes in its first excited state. Hence, the revised valence shell configuration of Cl is 3s² 3p⁴ 3d¹. This indicates that for the formation of ClF₃, Cl expands its octet and Cl has 10 electrons in ClF₃.