The correct option is B Carbonic anhydrase
When carbon dioxide diffuses into RBCs (erythrocytes), it combines with water, forming carbonic acid. Carbonic acid is unstable and quickly dissociates into H+ ions and bicarbonate ions. The reaction occurs in the presence of enzyme carbonic anhydrase which is found in RBCs.
Carbonic anhydrase enzyme is not found in WBCs (leucocytes), tissue fluid or platelets (thrombocytes).