【正确答案】 C

【答案解析】Diction; Rhetorical construction
The wording is unnecessarily awkward. Also, "able" suggests agency, which is not appropriate with the subject methods.The phrase cannot be used would be preferable to are not able to be used.
A The wording is awkward for the reason given above.
B The wording is awkward, both for the reason given above and because of the phrase in the future of insufficient cashflow.
C Correct. This version uses the preferable cannot successfully be used and is otherwise: clearly and concisely worded.
D Cannot be used without succeeding in creating a problem is badly worded. Surely the point of the sentence is that the methods cannot be used successfully without creating a future problem of insufficient cash flow; as worded, this version seems to suggest that successfully creating a cash flow problem is a prerequisite for being able to use the methods. Also, in the future of insufficient cashflow is awkward.
E As already indicated, this version seems to suggest that successfully creating a cash f ow problem is a prerequisite for being able to use the methods.
The correct answer is C.