Final answer:
The closest word in meaning to 'confidential' is 'private'.
The word in the sentence that is closest in meaning to 'confidential' is private (option D).
Step-by-step explanation:
The word 'confidential' means something that is meant to be kept secret or not shared with others. In the given sentence, the memo was accidentally sent to all the members of the school board, which means it was no longer private or confidential.
For example, if you have a diary and you write personal thoughts and secrets in it, the diary is your private space where you keep information in a confidential manner that is not meant to be shared with others.