Should I put my resident state California if I lived there all my life until August 2023? Or should I put Montana which I moved to in Augusst 2023?

[xmasbaby0]

In MY INFO use the mailing address you want the IRS to use if they need to mail you a letter or a check.  As for your tax returns.....you will need to prepare a part-year return for CA and a part-year return for MO.

https://ttlc.intuit.com/questions/1901227-how-do-i-file-a-part-year-state-return

https://ttlc.intuit.com/questions/1901560-how-do-i-file-if-i-moved-to-a-different-state-last-year

https://ttlc.intuit.com/questions/2895983-how-do-i-allocate-split-income-for-a-part-year-state-retur...

[TomD8]

Enter MT as your State of Residence in Personal Info, and indicate that you also lived in CA.  Enter the date that you began living in MT.