B) 2

Explanation:
When an SDS-PAGE gel is run under reducing conditions, the disulfide bonds that hold the protein subunits together are broken by the reducing agent, typically beta-mercaptoethanol or dithiothreitol. As a result, the protein subunits separate and migrate independently on the gel.

- SDS-PAGE under reducing conditions
In SDS-PAGE, sodium dodecyl sulfate (SDS) is used to denature the proteins and give them a negative charge, allowing them to migrate towards the positive electrode. The reducing agent breaks the disulfide bonds, which are responsible for maintaining the quaternary structure of the protein.

- IgG antibody structure
IgG antibodies are composed of two heavy chains and two light chains. Under non-reducing conditions, the disulfide bonds between the heavy chains are intact, resulting in a single band on the gel. However, under reducing conditions, the disulfide bonds are broken, and the heavy and light chains separate.

- Migration of IgG antibody subunits
The heavy chains of the IgG antibody are larger and migrate more slowly on the gel, while the light chains are smaller and migrate faster. Therefore, when the IgG antibody is run on an SDS-PAGE gel under reducing conditions, two distinct bands are observed - one corresponding to the heavy chains and the other to the light chains.

- Staining with Coomassie blue
Coomassie blue is a commonly used protein stain that binds to proteins and allows them to be visualized on the gel. When the gel is stained with Coomassie blue, the two bands corresponding to the heavy and light chains of the IgG antibody will be clearly visible.

Therefore, when a pure IgG antibody is run on an SDS-PAGE gel under reducing conditions and stained with Coomassie blue, two bands will be seen - one for the heavy chains and one for the light chains. Thus, the correct answer is option b) 2.