__________ you work hard, you will not succeed.

The correct answer is B: Unless. 'Unless' is a conjunction that means 'if not' or 'except if.' It introduces a condition that, if not met, will lead to a specific outcome. In the given sentence, '__________ you work hard, you will not succeed,' 'unless' perfectly conveys the meaning that success will not be achieved if the condition of working hard is not fulfilled. Essentially, it means 'If you do not work hard, you will not succeed.' A: If would create a different meaning, implying 'If you work hard, you will not succeed,' which is illogical in this context. C: When indicates a time, not a condition, and would mean 'At the time you work hard, you will not succeed,' which doesn't fit. D: While suggests simultaneity or contrast, neither of which applies here. Therefore, 'unless' is the only word that correctly establishes the necessary negative conditional relationship between working hard and achieving success, highlighting the importance of effort.