Why would you expect to find argon in its pure uncombined form in nature?

Argon has a full octet of outer electrons. It is thus in it's most energetically stable state. It would be very energetically unfavorable (almost impossible to date) to even form compounds. So in nature it would only be found in it's pure uncombined form.

My answer is letter d

Answerair masses develop near the equator are warm; tropical air mass