Final answer:
Argon (Ar) is the atom that has a Lewis structure with an octet.
Step-by-step explanation:
The atom that has a Lewis structure with an octet is C. Ar (Argon).
Argon (Ar) is in group 8A of the periodic table, which means it already has a full octet of valence electrons (8). Since it has a full octet, it does not need to gain or lose any electrons to achieve stability.
In contrast, atoms like Cl, Na, and Mg do not have a full octet and need to gain or lose electrons to achieve stability.