The missing word is "for" or "since". The sentence can be completed as: "For a long time, this city has been prosperous," or "Since a long time, this city has been prosperous," to indicate the duration of the city's prosperity.
______ a long time this city has been prosperous.
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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.