The correct option is C Chloride shift
During internal respiration, HCO−3 ions diffuse out into plasma and Cl− ions enter into the RBCs at the level of tissues. This is known as hamburger shift. This process occurs for maintaining the electrostatic neutrality of the plasma. When oxygenated blood becomes de-oxygenated, then the chloride content of RBCs increases. That's why it is known as chloride shift. At the level of external respiration or alveoli, Cl− move out as HCO−3 move in, this is called reverse of chloride shift.