"Kem Cho" is a common greeting in the Gujarati language, spoken in the Indian state of Gujarat. It is similar to asking "How are you?" or "How's it going?" in English.
When someone says "Kem Cho," an appropriate reply would be to reciprocate the greeting and answer how you are doing. In Gujarati, you can say "Majama" (pronounced "maa-ja-maa"), which means "I'm fine" or "I'm doing well." Alternatively, if you are not familiar with Gujarati, a simple "hello" or "hi" would also work as a polite response.