The correct distinguishing feature is:
A postulate is assumed to be true, and it does not have to be proven.
Here's the reasoning for the other options:
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A conjecture is a statement believed to be true based on some evidence but has not yet been proven.
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A definition is a precise explanation of a term or concept, outlining its meaning and necessary conditions.
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A theorem is a statement that has been proven to be true based on previously established theorems, postulates, and definitions.
Therefore, the accurate feature among the given options is related to postulates.