User Acceptance Testing (UAT) is a critical phase in the implementation process where the system is tested by the actual business users to ensure it meets their requirements and works as expected. The Business Analyst (BA) should participate to facilitate the process and address any issues that arise. Key stakeholders need to be involved because they have a vested interest in the outcome and can provide valuable feedback. Most importantly, business users should participate because they are the end users of the system, and their approval is crucial for a successful implementation. Therefore, the correct option is BA, business users, and key stakeholders.