This is a Fill In The Blank question, so look for the answer that fits most clearly within the scope of the argument. Testing for PSA is beneficial because it diagnoses prostate cancer very early on, even before symptoms are present. Based on the structure of the last sentence, there must be some reason why PSA testing is not beneficial for men over 75. Consider each choice in turn:

(A) This is irrelevant; presumably, men who already have prostate cancer would not be tested for early signs of prostate cancer.
(B) It is not clear from this choice whether additional tests are so taxing that it isn't worth identifying and possibly treating prostate cancer; one suspects the opposite is the case.
(C) This is correct. Based on the passage, it would appear that if prostate cancer is diagnosed only by PSA testing, it is in a very early stage. It would likely take some time before it becomes symptomatic. If someone is unlikely to develop symptoms, there is no reason to test for, and possibly treat, the cancer.
(D) This choice is not specific to men over 75; in fact, the false-positive rate is already mentioned in the passage.
(E) Like (B), it isn't clear that this drawback is enough to make testing not worthwhile. It also isn't directly relevant to the claim that men over 75 "would probably not benefit"--they may well benefit, even if the test is very expensive.