I'm not sure you necessarily have to mention the C. But depending on how many GCSEs you have, they may wondering why you have less GCSEs if you leave it out. Anyways, a C is a good pass, and definitely worth mentioning on your cv! What I usually do is put maths/english at the top of the list (as that is what they are most interested in usually), then I put all the best grades near the top of the list, and put the rubbish ones right down the bottom lol. C is nothing to be ashamed of. Perfectly good grade. Id mention it, if anything it means youve done more GCSEs than if you dont, and thats better!!
All the GCSE results for the same exam board taken in the same exam season appear on the same certificate. If you need to show it to prove your A grades then they will see the C, so be honest from the start.
What is the CV for?
Most people would just put 11 GCSE A* to C including English and Maths. For UCAS every grade must be declared.
I don't want to put 11 gcse A to C because they'll assume I got 1 A 2 Bs and the rest Cs