To answer this question, the user needs to understand what measures can be used to assess test progress.

Option A: Number of undetected defects. This is not a valid measure of test progress because it does not indicate how many tests have been executed or how many defects have been found and fixed. This metric only counts the number of defects that have not been found yet.

Option B: Total number of defects in the product. This is not a valid measure of test progress because it does not indicate how many defects have been found and fixed. This metric only counts the total number of defects in the product, which can increase or decrease over time.

Option C: Number of test cases not yet executed. This is a valid measure of test progress because it indicates how many test cases are left to execute. As test cases are executed, this number will decrease, providing a clear indication of how far the testing effort has progressed.

Option D: Effort required to fix all defects. This is not a valid measure of test progress because it focuses on the effort required to fix defects rather than the progress made in finding and fixing defects. This metric does not provide any information about the state of testing or how close the team is to completing the testing effort.

Therefore, the correct answer is:

The Answer is: C. Number of test cases not yet executed.