The correct answer is B. GridView control in ShoppingCartContent.ascx uses ContentTable of ShoppingCartInfo as data source - see ReloadData() method in code behind. TotalTax column of this table contains tax values for individual items.
Property ShoppingCartInfo.TotalTax contains the sum of taxes for all cart items.
And finally, ShoppingCartInfo.ContentTaxes DataSet serves as a cache for tax classes applicable to (some of) shopping cart items. One will probably only use this in some cases of taxes-related customizations.