It is from official standard English test. The question is like below.
Fitness enthusiasts may feel that their workouts are no longer as effective as they were. When an individual ______________ for a while, the body experiences a "plateau effect," which can be overcome through changes in the workout.
(a) is exercising (b) will have been exercising (c) has been exercising (d) exercises
According to a book, the answer is (c). However, the original test-offering institution has never revealed their answers but they only sold the copyright to use their test questions for making books. So the answer offered by the book may not be correct.
Of course (b) doesn't make sense. I personally think (d) could be also good, but the test's criterion makes it clear that they, at this level, check only the degree of understanding with regard to continuous tenses. The book rules out (d) because simple present tense is not the area that this test evaluates. Anyway, the reason for the book's choice of (c) over (a) is because the phrase of period following for often goes with perfect continuous tenses. But I think I heard somewhere that present perfect doesn't go with the conjunction, when. You know, when refers to a certain point in timeline, whereas present perfect has more to do with duration. So, I thought present perfect continuous would also not go well with the conjunction, when. So, I thought (a) might be also possible. I want to know what should be the only one correct or the most desirable answer among (a), (c), and (d), and why. Thank you for your help :)