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The term 'sage' as an adjective means wise or possessing great wisdom. Therefore, the synonym that best fits this definition is 'sagacious,' which also indicates having good judgment and being wise. Both words share a common root meaning related to wisdom and insight, making 'sagacious' an appropriate choice.

The other options do not align with the meaning of 'sage.' For example, 'foolish' denotes a lack of wisdom, 'naive' refers to a lack of experience or sophistication, and 'illogical' describes something that does not conform to rational thought. These contrasting meanings highlight why 'sagacious' is the only correct synonym in this context.