If a latecomer joins the congregation during Ruku' and bows with the Imam before the Imam raises his head, what is the status of that rak'ah?

Catching Obligatory Prayers Part 2: Sunnah Alignment

• They have missed the rak'ah.
• They have successfully caught that rak'ah.
• They must repeat the entire prayer.
• The rak'ah is counted as voluntary.