**Core Concept:** Alkali resistant haemoglobin refers to a type of haemoglobin that maintains its ability to bind oxygen under alkaline conditions. This property is crucial for the transport of oxygen in bodily fluids with a higher pH, such as the lungs and gastrointestinal tract.

**Why the Correct Answer is Right:** The correct answer is **D.** Hemoglobin A2, also known as delta-haemoglobin, is a variant of haemoglobin with a higher affinity for oxygen at alkaline pH levels. This variant is formed due to the substitution of amino acid valine for tyrosine at the beta-globin chain position 6. Hemoglobin A2 contributes to the overall oxygen-carrying capacity of the blood and ensures efficient oxygen binding under alkaline conditions.

**Why Each Wrong Option is Incorrect:**

A. Hemoglobin F (alpha-globin chain position 14-15) is a variant of haemoglobin found in newborns and is not alkali resistant.
B. Hemoglobin M is a group of abnormal haemoglobins characterized by increased oxygen affinity and is not alkali-resistant.
C. Hemoglobin S is a variant of haemoglobin causing sickle cell disease, which has no relation to alkaline pH resistance.

**Clinical Pearl:** Understanding alkali-resistant haemoglobin is essential for clinical scenarios involving acid-base imbalances, such as in the context of respiratory alkalosis, when a higher pH in the blood leads to increased oxygen affinity and can affect the distribution of haemoglobin variants.

**Correct Answer:** **D.** Hemoglobin A2 is an alkali-resistant haemoglobin variant that maintains its oxygen-binding affinity under alkaline conditions, ensuring effective oxygen transport in tissues and organs with higher pH values.