In formal writing, which form correctly completes the phrase: The winner was announced by someone ___ was not present?

The correct form to complete the phrase "The winner was announced by someone ___ was not present" is "who." This is because "who" is the appropriate relative pronoun to refer to a person, making it suitable for describing a subject in a sentence.

Using "who" in this context indicates that the clause is providing additional information about a specific individual (the someone) and their state of presence. It emphasizes the person's role or identity related to the action of announcing the winner.

The other options are not suitable in this context: "whom" is used as an object rather than a subject, which does not fit with the sentence structure here; "that" can refer to people but is generally used in restrictive clauses and does not convey the same level of specificity about the subject in formal writing; and "which" is typically used for non-human antecedents or additional, non-essential information about the subject. Therefore, "who" is the best choice to maintain clarity and grammatical correctness in the sentence.