Final answer:
The common factor throughout the Greek city-states was language, as all Greeks spoke dialects of Greek that were mutually intelligible.
Step-by-step explanation:
The aspect that was the same throughout the Greek city-states was C. language. Each Greek polis, or city-state, was indeed unique in its own laws and currency, but the citizens across these city-states communicated through mutually intelligible dialects of the Greek language. This shared language fostered a sense of Greek identity and facilitated cultural exchange, as seen in Panhellenic festivals like the Olympics. Despite their independent nature and frequent conflicts, ancient Greeks considered themselves part of a larger Hellenic world, linked by language, religion, and cultural practices.