* Valence electrons are the electrons in the outermost shell of an atom. These electrons are involved in chemical bonding.

* Chlorine is in the third row (period) of the periodic table. This means it has three electron shells.

* The electron configuration of chlorine is 2, 8, 7. This means it has 7 electrons in its outermost (third) shell.

Let's break down the other options:

* A) Nucleus: The nucleus contains protons and neutrons, not electrons.

* B) First shell: The first shell can only hold 2 electrons.

* C) Second shell: The second shell can hold 8 electrons.

Therefore, the third shell is where chlorine's valence electrons reside.