TDA Rule 51-B applies here. Since C is not facing the opening bet, at TDs discretion C may be allowed to "forfeit the amount of the intended undercall and fold". The intended undercall is 2600 here. Note it's TDs discretion whether to allow the fold or to require a full call.
Regarding the amount that C would forfeit if allowed, should it be 2600 or 5000? This question was discussed at the 2019 Summit. The Board members were somewhat split in initial discussion but on the floor the "intended undercall" was adopted.