Please help with the formula

[Hanna_Kirana_Ap]

I have column A with the breakdown option and have columns B and C, what is the formula if someone chooses A1,A2 it will link to column B and if choose A3,A4 it will link with column C?

[TheTimeSavingCo]

You'll need to use an automation for that.  An example would be making it trigger whenever column A is updated, and using conditional logic to update column B and C as required

I recommend checking out the automation documentation