Which box 12 abcd?

The coding in box 12 of an IRS form W-2 mean different things.  This TurboTax Help explains the different codes.

TurboTax supports multiple box 12 codes that may be reported on your IRS form W-2.

It doesn't matter which box  12a-d you enter it in.  W2 Box 12a - 12d are just the box numbers. They don't have to be filled in. But if they have a Code (1 or 2 capital letters) and amount you need to enter it. Don't confuse the lowercase letters a-d with the uppercase Codes A, B, C, D, DD, etc. The lowercase a-d are just part of the box label. You don't enter them. You can enter the Code and Amount in any box 12.