Here's why:

* Electronic Configuration: Chlorine's electronic configuration is 1s² 2s² 2p⁶ 3s² 3p⁵.

* Valence Electrons: Valence electrons are the electrons in the outermost shell of an atom. For chlorine, the outermost shell is the third shell (n=3).

* Subshells: Within the third shell, there are the 3s and 3p subshells.

* Filling Order: The 3s subshell is filled first, followed by the 3p subshell. Since chlorine has 7 electrons in its third shell, the 3p subshell contains the valence electrons.

Therefore, the 3p subshell contains the valence electrons of chlorine.