What is the synonym of who?

The best synonym for "who" depends on the context and the grammatical function of the word. Here are some possibilities:

For the subject of a verb:

* Which (when referring to a person or thing)

* Whom (formal, used as the object of a verb or preposition)

* What (when referring to a thing)

* That (when referring to a person or thing, often used in restrictive clauses)

For the object of a verb or preposition:

* Whom (formal)

* That (informal, sometimes used in restrictive clauses)

For a relative pronoun in a defining clause:

* That (informal)

Examples:

* Who is coming to dinner? -> Whom are you inviting to dinner?

* I know who stole the cookie. -> I know that you stole the cookie.

* Which of you is the tallest? -> What is the capital of France?

It's always best to consider the specific context and the grammatical function of "who" before choosing a synonym.