How do we export responses for multiple choice questions in single column , comma separted (via api)

You can not do this if your export is JSON or NDJSON
In the body of the API call, use the parameters:
breakoutSets = False (comma separate in one column)
useLabels = True (show text not values)
Here are all the options:
https://api.qualtrics.com/api-reference/b3A6NjEwMzk-start-response-export