The correct option is C decreases the affinity of haemoglobin to oxygen but increases its affinity to hydrogen ions.
In the tissues, high concentrations of carbon dioxide decreases the affinity of haemoglobin to oxygen so that it can dissociate. It increases the affinity of haemoglobin to hydrogen ions, so it can remove the excess of hydrogen ions from solution, which are formed when the carbon dioxide reacts with water and forms carbonic acid.