For my HSA contribution which Box 12 do I use a or b?

[DavidD66]

If you are preparing a Form W-2, you can put your HSA contribution in box 12 a, b, c, or d.  It doesn't matter, which box 12 item goes in which box. Just use them in order.  If you are entering a W-2, enter it the same way it is printed. 

[Mike9241]

But make sure you enter the code W, which designates this line as being for an HSA

[dmertz]

On TurboTax's W-2 form, enter your W-2 exactly as received.  The amounts shown in box 12 of your W-2 are not  to be entered anywhere else in TurboTax.  TurboTax will automatically pick up from your W-2 entry your HSA contribution that is enteredthere with code W.  Do not enter that amount separately in TurboTax's HSA section.