**Core Concept**
The question is testing the understanding of the molecular structure of hemoglobin (Hb), a protein in red blood cells responsible for transporting oxygen. Hemoglobin is a tetrameric protein, composed of four polypeptide chains (two alpha and two beta chains) that bind to four iron atoms, each coordinated with a heme group.

**Why the Correct Answer is Right**
The correct answer is based on the molecular structure of hemoglobin. Each subunit of hemoglobin contains one heme group, which is a planar ring structure composed of a porphyrin ring and an iron atom (Fe++). The iron atom is crucial for binding oxygen to hemoglobin. In each heme group, the iron atom is coordinated with a proximal histidine residue, an oxygen molecule, and a distal histidine residue. The four heme groups in hemoglobin are arranged in a symmetrical manner, with each heme group binding one oxygen molecule.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the correct number of iron atoms in a hemoglobin molecule.
* **Option B:** This option is incorrect because it implies that there are two iron atoms in a hemoglobin molecule, which is not the case.
* **Option C:** This option is incorrect because it implies that there are four iron atoms per heme group, which is not correct.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the binding of oxygen to hemoglobin is a cooperative process, meaning that the binding of one oxygen molecule to one heme group increases the affinity of the other heme groups for oxygen. This is due to the conformational changes in the protein that occur upon oxygen binding.

**Correct Answer:** 4. Each hemoglobin molecule contains four iron atoms, one in each heme group.