Final answer:
The correct type of maintenance testing for system decommissioning is Data migration testing, which ensures data integrity and successful transfer to a new system.
Step-by-step explanation:
When a system is targeted for decommissioning, the appropriate type of maintenance testing required is C. Data migration testing. This process ensures that all essential data is accurately transferred from the old system to the new environment without loss or corruption. During decommissioning, testing is crucial to validate the integrity of data after transfer, to make sure that all necessary data has been moved successfully, and to certify that the new system can utilize that data correctly. Retirement testing would be less about maintenance and more about the process of actually taking the system offline. Regression testing and patch testing are typically performed during active development and maintenance, not as part of a decommissioning process.