A.
This sentence would be acceptable in informal conversation, but in careful writing, it is not correct. The problem is that indefinite pronouns like "everyone" are singular (We would never use a plural verb with them.), but "they" is plural. We need to be consistent in pronoun number. A word can't be plural in one place in the sentence and singular elsewhere in the same sentence.
Also the sentence ends with a preposition. Some people would consider that to be incorrect.