Concept:"Everyone but one" means all people except one.
Here, "but" is used as a preposition meaning "except".
So the sentence states that only one person did not oppose the proposal.
Explanation:The singular verb "was" requires a singular subject.
"Everyone" is singular and means "every person".
"All" is plural, so it would need "were" instead of "was".
"Anyone" does not fit because the phrase means "any single person", not "all except one".
Thus, "Everyone but one" correctly means all except one person were against the proposal.
Answer:A. Everyone