In general, you never want to specify a Codeset for a Code 128 barcode. Code 128 encoding follows a set of rules that allows the barcode to change codesets mid stream. That is, a code 128 barcode should never be limited to a single codeset. Rather, it should change between codesets as required to make the most compact and efficient barcode. Thus, if the data being barcoded only contains characters from Subset C, the resultant Code 128 barcode in Wasp Labeler will be a Code 128 Subset C barcode.
But if you do need to force a certain subset, Code 128 A B and C are all selectable from the Symbology drop down menu on the Barcode tab of the object's Properties window.