**Core Concept**
Hemoglobin is a tetrameric protein in red blood cells that plays a crucial role in oxygen transport. It consists of four polypeptide chains (two alpha and two beta chains) and contains a heme group, which is responsible for binding oxygen.

**Why the Correct Answer is Right**
Each hemoglobin subunit contains a heme group, which is a prosthetic group consisting of an iron ion (Fe2+) coordinated to a porphyrin ring. The porphyrin ring is responsible for binding oxygen, and the iron ion is the site of oxygen binding. The presence of the heme group is essential for the oxygen-carrying capacity of hemoglobin.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as it does not provide a specific number of prosthetic groups.
**Option B:** This option is incorrect as it suggests that there are four prosthetic groups, which is not accurate.
**Option C:** This option is incorrect as it suggests that there are two prosthetic groups, which is half the actual number.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that each hemoglobin subunit contains a single heme group, and there are four subunits in total, making a total of four prosthetic groups in a hemoglobin molecule.

**Correct Answer: C. 4**