A valid reason for writing test cases based on experience and intuition is C. Tests based on experience and intuition can supplement formal techniques. Experience and intuition are valuable sources of test ideas that can help testers to identify potential risks, errors, or scenarios that may not be covered by formal techniques. Formal techniques are systematic methods that use rules or algorithms to derive test cases from the test basis (such as specifications, code, etc.). Formal techniques include black-box techniques (such as equivalence partitioning, boundary value analysis, etc.) and white-box techniques (such as statement coverage, decision coverage, etc.). Formal techniques can provide a high level of coverage and consistency, but they may not be able to capture all possible situations or behaviors of the system or component under test. Therefore, tests based on experience and intuition can supplement formal techniques by adding more diversity and creativity to the test cases. A detailed explanation of experience-based testing techniques can be found in [A Study Guide to the ISTQB® Foundation Level 2018 Syllabus], pages 74-76.