HSA contribution/ distribution is which box 12 on W2 a,b,c?

Boxes 12a, 12b, 12c, and 12d on the W-2 are just four available spaces that can each contain a code and an amount. HSA contributions could be in any one of the four boxes, but it will have a code W before the amount. The W identifies it as HSA contributions.

HSA distributions are not shown on your W-2. HSA distributions are reported on a Form 1099-SA.

HSA contributions are on your W-2, box 12 code W.   Distributions are reported to you on 1099-SA.  

• The UPPER-CASE (capital) letters in box 12 stand for different things and are listed in the link below. These are the letter codes you'll need to enter into TurboTax
• The lines labeled 12a, 12b, 12c, and 12d (sometimes listed as 12a-d) on your W-2 are just line labels, not codes.   Don't enter these line labels. For example, if you see "D" on line 12a, enter "D" in TurboTax, not "a". And if line 12a is blank, don't enter anything

Box 12 codes