Final answer:
Ancient Greek city-states were united by a common language, religion, and cultural practices, particularly their shared belief in the Greek pantheon. The correct answer is options c,d.
Step-by-step explanation:
The independent city-states of ancient Greece, although politically disunited, were held together by several key factors including their language, religion, and shared cultural practices. Remarkably, these diverse city-states, or poleis, identified culturally and spiritually as Greeks - partaking in common practices such as reciting epic poems like the Iliad and Odyssey and engaging in symposia.
One of the most unifying elements was the belief in the Greek pantheon, with worshippers across the region venerating the gods at sacred sites like Delphi and Olympia and competing in Panhellenic festivals such as the Olympic Games, which honored Zeus. Thus, among the options provided, 'd. religion - the beliefs in the Greek pantheon' is the correct option that historically held the independent city-states of ancient Greece together.