Final answer:
Class 4 flight instructors are required to perform progress checks as specified by the supervising instructor, which must be done at least before the student's first solo flight and before any flight test.
Step-by-step explanation:
Flight progress checks for Class 4 instructors must be completed as stipulated by the supervising instructor, but there are key milestones when these checks are absolutely required. According to Option B, these progress checks must occur at a minimum before a student's first solo flight and before any flight test. This ensures that the student is adequately prepared and that the instruction provided meets the necessary standards for safety and competence.