The full sentence should be, "He is one of my best friends". This is because "one of" implies that he is part of a larger group (friends), and the noun that follows should be plural.
He is one of my best ______?
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Question context
This question is part of 🗣️ Verbal Ability within the broader 🧠 Aptitude Mastery subject and appears in the Audit preparation collection. Reviewing the answer together with nearby questions helps reinforce the underlying concept and identify how it may appear in competitive exams.