To answer this question, we need to understand the different test activities and when faults can be found most cost-effectively.
Option A) Execution - This option is incorrect because the execution phase involves actually running the tests and checking if the system behaves as expected. While faults can be found during execution, this phase typically involves more effort and resources, making it less cost-effective compared to other activities.
Option B) Design - This option is incorrect because the design phase involves creating test cases and test scenarios based on the system requirements. While faults can be identified during the design phase, it is generally more cost-effective to find and address faults earlier in the testing process.
Option C) Planning - This option is correct because the planning phase involves defining the test objectives, test strategy, and test approach. This is the phase where potential faults or defects can be identified and addressed early on, allowing for more cost-effective testing. By planning effectively, resources can be allocated efficiently, and potential risks can be identified and mitigated.
Option D) Check Exit criteria completion - This option is incorrect because checking exit criteria completion is a part of the test closure activities. While it is important to ensure that all exit criteria have been met before concluding the testing process, it does not directly relate to finding faults cost-effectively.
Therefore, the correct answer is Option C) Planning. This option is correct because during the planning phase, potential faults can be identified and addressed early on, leading to more cost-effective testing.