Which has the least stable configuration?

NaCl

H2O

A single atom of Chlorine

A single atom of Argon

Answer is: a single atom of chlorine.

Chlorine has electron configuration: ₁₇Cl 1s²2s²2p⁶3s²3p⁵.

According to his electron configuration, chlorine needs one electron to achive noble gas configuration (most stable because valence shell is filled).

NaCl, H ₂O and Ar have stable electron configuration.