のは誰なの

who is it that ...

→ の
phrase Rare #20,514
who is it that ... Used after an action or description to ask who did it or fits the description. Often used in casual questions.
JA: この絵を描いたのは誰なの?
EN: Who was it that painted this picture?

一緒に住んでいるのは誰なの?

Who do you live with?

あの会議を企画したのは誰なの?

Who organized that meeting?

雪だるまを作ったのは誰なの?

Who made the snowman?

ピザを全部食べたのは誰なの?

Who ate all the pizza?

トイレへ行きたいのは誰なの?

Who wants to go to the bathroom?

— どん底作家の人生に幸あれ! (2019)

やり直そうと電話してきたのは誰なの

Who called to try to start over?

— 8 1/2 (1963)

倉庫のドアの鍵をかけ忘れたのは誰なの?

Who forgot to lock the warehouse door?

最後にコンピューターを使ったのは誰なのだ。

Who was the last person to use the computer?

警察を呼んだのは誰なの?

Who called the police?