Help the detective complete her dramatic final report by choosing the grammatically correct option.

The mysterious informant _____ the detective confided turned out to be the mastermind all along!

The correct answer is in whom.

In formal English, prepositions (like "in") are placed before the relative pronoun. When referring to a person immediately following a preposition, you must use the objective case whom. You cannot use "who" or "that" right after a preposition in a relative clause!