It’s correct when written because acronyms are representing the words the abriviate. In this case an unidentified object. If you where to speak the acronym than just “a” is correct because you are saying the letter “U” not the word “unidentified”. Does that make sense. Different rules for written and spoken.
It’s correct when written because acronyms are representing the words the abriviate. In this case an unidentified object. If you where to speak the acronym than just “a” is correct because you are saying the letter “U” not the word “unidentified”. Does that make sense. Different rules for written and spoken.
All common English-language standards and style guides will tell you the article should be based on the pronunciation of the acronym.
E.g., https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Usage/faq0167.html