Concept:A monoatomic gas consists of single atoms that are not bonded to other atoms of the same element.Explanation:Argon is a noble gas, so it exists as isolated atoms written as Ar.Chlorine normally exists as diatomic molecules: Cl2.Nitrogen normally exists as diatomic molecules: N2.Oxygen normally exists as diatomic molecules: O2.Only argon fits the definition of a monoatomic gas.Answer:A. Argon