Transform 'You won't succeed unless you try harder' to use the word 'until'.

The transformation using the word "until" effectively conveys the condition under which success is possible, aligning it closely with the original meaning of the sentence. The phrase "You won’t succeed until you make an effort" indicates that success is contingent upon making an effort, which parallels the idea in the original statement that trying harder is a prerequisite for success.

This choice succinctly retains the original meaning by clearly stating that one must take an active step—making an effort—before success can occur. The use of "until" appropriately sets up the relationship between trying harder and achieving success, signaling that success will only come after making that necessary effort.

In contrast, the other options do not maintain the original nuance of conditionality or may alter the meaning. For example, some options might imply that success is entirely dependent on a different action or remove the specific notion of effort and trying, which is central to the original statement's intent. Thus, the selected answer stands out for accurately preserving the original message while fulfilling the transformation requirement.