To maintain the flow of ideas in his research work, Omar must retain the idea that "been described as circular or cylindrical" (Option B).
The flow of ideas refers to the logical sequence of thought that a text must retain to remain coherent and easy to understand.
Notice that immediately following the missing portion, there is a description of how UFOs have been described historically.
Hence the most appropriate fill-in is (Option B). See the attached for the full question.
Learn more about flow of ideas at:
https://brainly.com/question/2612257
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