Regression testing is the process of re-running functional and non-functional tests to ensure that previously developed and tested software still performs correctly after a change. A release upgrade is a significant change that can inadvertently introduce defects ("regressions") into existing, stable functionality. Therefore, the primary testing activity after an upgrade is to execute a regression test suite to verify that core business processes are unaffected and continue to operate as expected, ensuring business continuity.

A. Integration tests: This type of testing focuses on verifying the interfaces and data flow between different system components, which is not the primary goal after an upgrade.

B. String tests: This is a specific type of end-to-end test for a business process. While parts of it may be in a regression suite, it's not the correct overarching term.

C. User acceptance tests: UAT is typically performed to validate that new functionality meets user requirements before a go-live, not specifically to check for regressions in existing functions post-upgrade.

1. SAP Activate Roadmap Viewer for SAP S/4HANA Cloud, public edition: In the "Run" phase, the task "Perform Upgrade and Regression Testing" explicitly details this activity. The description states, "The purpose of this task is to perform the regression testing for the customer's existing business processes after the release upgrade in the quality system." (Access via SAP Activate Roadmap Viewer, specific task ID may vary with roadmap updates).

2. SAP Help Portal - SAP S/4HANA Cloud: The documentation on testing strategy, specifically "Test Your Processes in SAP S/4HANA Cloud," states: "After an upgrade, you need to perform regression tests to ensure that your business processes still run as expected. The SAP S/4HANA Cloud Test Automation Tool is the recommended tool for this purpose." (See SAP Help Portal, SAP S/4HANA Cloud, section on Implementation -> Test Your Solution).

3. Official SAP S/4HANA Cloud Documentation: The document "Testing in SAP S/4HANA Cloud" outlines the testing phases. It clearly positions Regression Testing as the key activity during the quarterly release upgrade cycle to ensure the stability of productive business processes. (Document available on the SAP Learning Hub for course S4C01e).