The answer they are looking for is Greeks.
In reality the Celts had as much as the Greeks in that. Celts had limited Democracy and it has long been a cultural trait. The Greeks gave us more of the form rather than the drive for it. English common law which is based heavily on Celtic law is the foundation of modern human rights and Democracy and a huge influence on the writers of the US Constitution. Greek and Roman Democracies were secondary influences and helped us in how we formated a Democracy right down to having a Senate like Rome did.