**Core Concept:**

The core concept being tested in this question is the definition of a haemoprotein and its distinguishing features from non-haem proteins. Haemoproteins are proteins containing haem, a prosthetic group consisting of a central iron atom bound to a porphyrin ring. They play crucial roles in cellular respiration, oxygen transport, and other biological processes.

**Why the Correct Answer is Right:**

The correct answer is **D**, which states "Non-heme iron containing proteins." Non-heme iron is a type of iron found in proteins, unlike haemoproteins which contain haem. Non-heme iron proteins lack the haem prosthetic group, and their functions are distinct from haemoproteins. For example, ferritin, transferrin, and carbonic anhydrase are non-heme iron containing proteins, while haemoglobin and myoglobin are haemoproteins.

**Why Each Wrong Option is Incorrect:**

Option A, "Proteins containing haem," is incorrect because it includes both haemoproteins and non-heme iron containing proteins. Haemoproteins have haem as a part of their structure, while non-heme iron proteins lack haem.

Option B, "Proteins containing heme," is incorrect because heme is a porphyrin ring containing iron, not a complete prosthetic group like haem. Heme is the component of haemoproteins, but not a characteristic feature of non-heme iron proteins.

Option C, "Proteins containing heme," is similar to Option B and is incorrect for the same reasons. Heme is a porphyrin ring containing iron but is not a complete prosthetic group like haem.

**Clinical Pearl / High-Yield Fact:**

Understanding the distinction between haemoproteins and non-heme iron containing proteins is crucial in medical practice as it helps differentiate between various proteins in physiological processes like oxygen transport, iron storage, and enzymatic functions. This distinction aids in diagnosing conditions like sideroblastic anemia, where abnormal haem synthesis results in ineffective erythropoiesis and impaired oxygen transport.