I wrote πιο κοινο and was marked wrong. The two correct answers given were
-πιό δημόσια. -Περισσότερο κοινό
γιατι οχι πιο κοινο;
If you use public as a noun, it's κοινό (in the sense of people in general). If you use public as an adjective it's δημόσιος (in the sense of accessible to all). More for nouns is περισσότερο, whereas for adjectives the comparative is πιο. Πιο κοινό would mean that κοινό is an adjective, which would mean "more common".
aha, thank you very much. Very helpful as I never thought it out before as I have used this term for so long both ways. :-)
Ι find the intended meaning of the English a bit hard to guess ... maybe the question could be reworded to make it clearer whether a noun or an adjective is wanted? (someday ... I know that designing these lessons must surely be very hard!)