Maintenance testing is important after migration, retirement or enhancement of an existing system. defines maintenance testing as follows:
Maintenance testing is a type of software testing that occurs after changes have been made to a system in order to ensure that no new defects have been introduced and that the system still meets its original requirements and specifications.
Regression testing (A) is important after any change in the system that may affect existing functionality.
Operational acceptance testing (B) is important before deploying a system into production environment.
System testing is important after integrating all components of a system into a complete system.