Reduction of blood pH will

The correct option is "decrease the affinity of haemoglobin with oxygen".

To understand why this is correct, we can analyze the relationship between blood pH, carbon dioxide levels, and oxygen transport. This biological phenomenon is known as the Bohr effect.

When there is an increase in metabolic activity (for example, in actively contracting muscles), cells produce more carbon dioxide (CO₂) as a byproduct of cellular respiration. This carbon dioxide diffuses into the blood plasma and red blood cells.

Within the red blood cells, carbon dioxide reacts with water (H₂O) to form carbonic acid (H₂CO₃), a reaction catalyzed by the enzyme carbonic anhydrase:
CO₂ + H₂O ⇌ H₂CO₃

Carbonic acid is unstable and rapidly dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-):
H₂CO₃ ⇌ H⁺ + HCO₃^-

The release of these hydrogen ions (H⁺) increases their concentration in the blood, which directly results in a decrease in blood pH (making the blood more acidic).

These hydrogen ions then bind to specific amino acid residues in the polypeptide chains of the hemoglobin molecule. The binding of H⁺ ions alters the conformational structure of hemoglobin, stabilizing its "T-state" (deoxygenated state or tense state). This structural change reduces the affinity of hemoglobin for oxygen.

Consequently, oxygen is more readily released (unloaded) from hemoglobin to the surrounding tissues that need it most. Therefore, a reduction of blood pH directly decreases the affinity of hemoglobin for oxygen, promoting oxygen delivery to metabolically active tissues.