There aren't many Greek words that start with the letter "C" in the modern English transliteration. The Greek alphabet doesn't have a "C". Words that appear to start with "C" in English are usually transliterated from a Greek word beginning with a K (kappa) or sometimes a Χ (chi).
Therefore, there's no single definitive answer. To give you an example that fits the criteria depending on the transliteration:
* Chaos (Χάος - Chaos): This is a well-known example. While it starts with "C" in English, its original Greek spelling begins with a Chi (Χ).