Thanks for the question! So (C) tells us that the flaw is that the argument overlooks the possibility that some cola not tested would’ve elicited a more favorable response than Sparkle Cola. But remember what the argument is saying. The argument just says that “Sparkle Cola elicits a more favorable response from consumers than any of the competing colas TESTED.” So since the argument limits itself to the other tested colas, how Sparkle Cola compares to untested colas doesn’t matter.
Hope this helps! Feel free to ask any other questions that you might have.