**Core Concept**
Hemoglobin synthesis is a critical process in erythropoiesis, the formation of red blood cells. It involves the coordination of multiple steps, including the production of heme and globin chains, which are then assembled into the hemoglobin protein.

**Why the Correct Answer is Right**
Hemoglobin begins to appear during the late stages of erythropoiesis, specifically during the orthochromatic erythroblast stage. At this point, the erythroblast has condensed its nucleus and is preparing for the final stages of maturation. The synthesis of hemoglobin involves the action of the enzyme ferrochelatase, which inserts iron into the heme molecule, and the globin chains are produced by the ribosomes. The combination of heme and globin chains forms the hemoglobin protein, which is essential for oxygen transport in the blood.

**Why Each Wrong Option is Incorrect**
**Option B:** This option is incorrect because the basophilic erythroblast stage is characterized by the presence of ribosomes and the synthesis of globin chains but not yet the formation of hemoglobin.
**Option C:** This option is incorrect because the polychromatic erythroblast stage is a transitional phase where the cell begins to lose its basophilic staining properties, but hemoglobin is not yet apparent.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that hemoglobin synthesis is a highly regulated process, and any disruptions can lead to anemia or other blood disorders. Understanding the different stages of erythropoiesis and the critical role of hemoglobin is crucial for diagnosing and treating various hematological conditions.

**Correct Answer: D. Orthochromatic erythroblast**