If a worshipper praying Zuhr alone has prostrated in the first unit, and the congregational Zuhr starts, how should they catch the congregation?

Catching Obligatory Prayers (إدراك الفريضة) Part 1

• Cut the prayer standing immediately.
• Add a second unit, perform Salaam (converting it to a 2-unit Nafal), and join the Imam.
• Complete three units and join.
• Do not join the congregation at all.