place Category Group on the filter shelf
Select Category C in the filter
Then build you viz with any detail you want - only the category C items are in the data - - just place the base dimension for the detail on the rows
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Use a calculation that is something like:
IF [New/Existing/Convert Wearers (copy)] = 'Category-C' THEN [Category Group]
ELSE [New/Existing/Convert Wearers (copy)]
That should return the lower level field (whatever it's called) when your group = 'Category-C', and otherwise just return the value from your grouped field.