To solve this problem, we need to understand the effect of a reduction in pH on blood and its components.$\newline$Let's analyze each option:$\newline \bullet$Option 1: Reduce the rate of heart beat.$\newline$ - A reduction in pH, indicating increased acidity, typically stimulates the heart to increase the rate of heartbeat to compensate for the decreased oxygen-carrying capacity.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 2: Reduce the blood supply to the brain.$\newline$ - A reduction in pH does not directly reduce blood supply to the brain. In fact, the body often increases blood flow to vital organs like the brain to ensure adequate oxygenation.$\newline$ - Therefore, this option is incorrect.$\newline \bullet$Option 3: Decrease the affinity of hemoglobin with oxygen.$\newline$ - A decrease in pH (acidosis) causes a rightward shift in the oxygen-hemoglobin dissociation curve, known as the Bohr effect.$\newline$ - This shift decreases the affinity of hemoglobin for oxygen, facilitating oxygen release to tissues.$\newline$ - Therefore, this option is correct.$\newline \bullet$Option 4: Release bicarbonate ions by the liver.$\newline$ - The liver does not release bicarbonate ions in response to changes in blood pH. Bicarbonate ions are primarily managed by the kidneys and the bicarbonate buffer system in the blood.$\newline$ - Therefore, this option is incorrect.$\newline$Based on the analysis, the correct option is Option 3: Decrease the affinity of hemoglobin with oxygen.