Final answer:
Oxygen is much more reactive than argon, as it readily forms compounds with other elements while argon, being a noble gas with a complete electron shell, has very low reactivity.
Step-by-step explanation:
When choosing between oxygen and argon in terms of reactivity, oxygen is the more reactive element. Oxygen's high reactivity is evident as it readily forms compounds with most other elements, which is a cornerstone of numerous chemical processes.
In the context of the reactivity series and periodic table trends, oxygen, being part of the chalcogen group, is known for its ability to gain electrons and form oxides, while argon, as a noble gas with a complete electron shell, exhibits very low reactivity due to its stable electron configuration [Ne]3s23p6.
This stability means that argon does not readily form chemical bonds, which is why we don't find compounds with argon under normal conditions.