What is a polis?
a.
a city-state run by kings
b.
the head of a greek city's government
c.
an elected official in the athenian assembly
d.
a small state run by citizens
a.?

Polis literally means "city" in Greek. Also, polis is a city state in ancient Greece, especially as considered in its ideal form for philosophical purposes. (City-state) So D is out.
There are still some city-states in the world today, like Monaco or Luxembourg. I think that's a polis.
So it's most likely A.