Deductions & credits

Lines 12a, 12b, 12c and 12d are not really separate lines on your W-2.  They are all "box 12" items.  Some employers use a, b, c and d when they need to list multiple things, but the order or lower case letter is not important, what matters is the capital letter code.  Select the code (W for HSA) and enter the amount.  Don't try to combine the letter c and code W, and don't use "c" as the code.