**Core Concept:** Erythropoiesis is the process by which red blood cells are produced in the bone marrow. Haemoglobin is the protein responsible for transporting oxygen in the blood.

**Why the Correct Answer is Right:** Haemoglobin first appears in the final stage of erythropoiesis, specifically in the late stage of erythroblast maturation. This stage is called the "polychromatophilic" or "polychromasia" stage, where the haemoglobin levels are highest and the red blood cells are most easily distinguishable under a microscope.

**Why Each Wrong Option is Incorrect:**

A. **"Stem cells" is not a stage in erythropoiesis but the starting point for the process.**
B. **"Proerythroblasts" is an early stage in erythropoiesis, before the haemoglobin production reaches a significant level.**
C. **"Megakaryocytes" are the cells responsible for platelet production, not haemoglobin production.**
D. **"Erythroblasts" is an intermediate stage in erythropoiesis, but haemoglobin production is not yet at its peak.**

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

Giemsa stain is commonly used in histopathology to differentiate between different stages of erythropoiesis in bone marrow. The term "polychromasia" comes from the Greek words "poly" (many) and "chroma" (color). The presence of polychromasia indicates increased bone marrow activity, often seen in conditions like megaloblastic anemia or recovery from anemia.

**Correct Answer:** D. Erythroblasts