Correct Answer: After user story definition and before coding starts.
Explanation:Acceptance criteria is typically defined in tandem with user story definition during release planning; however, acceptance criteria can also be defined during iteration planning once a story has been picked for the iteration. The one steadfast rule is that acceptance criteria must be defined before development begins. Like agile planning, the definition of acceptance criteria is constantly evolving as the conversation with the product owner matures. [The Art of Agile Development. James Shore.] [Planning, monitoring, and adapting]