**Core Concept**
Mercaptoethanol is a reducing agent that cleaves the disulfide bonds in immunoglobulin (Ig) molecules, resulting in the separation of the heavy and light chains. This process is essential in understanding the structure and function of antibodies.

**Why the Correct Answer is Right**
Mercaptoethanol works by breaking the disulfide bonds between the heavy and light chains of the Ig molecule, resulting in the formation of two separate chains. This occurs due to the reduction of the disulfide bonds (-S-S-) to sulfhydryl groups (-SH), which are then released as hydrogen sulfide gas. The heavy and light chains are then separated, allowing for further analysis of the antibody structure.

**Why Each Wrong Option is Incorrect**
* **Option A:** Mercaptoethanol does not split the Ig molecule into Fab and Fc fragments. This is incorrect as mercaptoethanol only breaks the disulfide bonds, whereas the production of Fab and Fc fragments requires proteolytic digestion.
* **Option B:** Mercaptoethanol does not split the Ig molecule into heavy and light chains only. While it does break the disulfide bonds, it also releases the Fab and Fc fragments.
* **Option D:** Mercaptoethanol does not split the Ig molecule into monomers. The Ig molecule is a dimer, and mercaptoethanol only breaks the disulfide bonds, not the intermolecular bonds holding the dimer together.

**Clinical Pearl / High-Yield Fact**
It's essential to note that mercaptoethanol can also be used to reduce the disulfide bonds in other proteins, such as enzymes and hormones. This is a useful tool in understanding protein structure and function.

**Correct Answer: C. Heavy and light chains**