The following statements about re-testing and regression testing are true:

I) Re-testing should be performed after a defect is fixed. Re-testing is a type of testing that verifies that a defect has been successfully resolved by executing a test case that previously failed due to that defect. Re-testing should be performed after a defect is fixed and delivered to ensure that it does not cause any new failures or side effects.

III) Re-testing and regression testing may be performed at any test level. Re-testing and regression testing are not limited to a specific test level, but can be applied at any level depending on the context and objectives. For example, re-testing and regression testing can be performed at unit level, integration level, system level or acceptance level.

IV) Regression testing may include functional, non-functional and structural testing. Regression testing is a type of testing that verifies that previously tested software still performs correctly after changes. Regression testing may include various types of testing depending on the scope and purpose of the changes. For example, regression testing may include functional testing to check if the software meets its requirements, non-functional testing to check if the software meets its quality attributes, or structural testing to check if the software meets its design or code standards. The following statement about re-testing and regression testing is false:

II) Regression testing should always be performed after a defect is fixed. Regression testing is not always necessary after a defect is fixed, as some defects may have a low impact or low likelihood of affecting other parts of the software. Regression testing should be performed after a defect is fixed only if there is a risk of introducing new defects or causing existing defects due to the changes made to fix the defect. Verified References: A Study Guide to the ISTQB® Foundation Level 2018 Syllabus - Springer, Chapter 2, page 19; Chapter 4, page 45.