Explanation
"Callow" means inexperienced or immature, so its antonym is "Veteran," which refers to someone seasoned and experienced. The other options—inexperienced person, diligent, and vigilant—do not directly oppose the meaning of callow.
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"Callow" means inexperienced or immature, so its antonym is "Veteran," which refers to someone seasoned and experienced. The other options—inexperienced person, diligent, and vigilant—do not directly oppose the meaning of callow.