A person is in the first rak'ah of Zuhr alone, has completed its first Sajdah, and congregation begins. What should be done?

When Congregation Begins during an Individual Fard or Sunnah Prayer

• Complete 2 rak'ahs, give Salaam, then join the Fard congregation.
• Break immediately even though a completed Sajdah has formed the first rak'ah structure.
• Complete all 4 before joining anything.
• Add 3 more and make it 5.