I think it's "KRA!"

Because coffee is a direct object of the sentence here, which calls for Accusative case. "One piją kawa", would indicate masculine noun "kaw" as the object they are drinking, which is not a word in Polish. ;)

Declension of kawa in wiktionary

Both are possible – English 'they' is not gendered, so can be all men, all women or mixed, whereas Polish differentiates 3rd person plural by grammatical gender.

If the option with „Oni” was not accepted(though I seriously doubt that's the case), you should definitely report that.