**Core Concept**
The question tests the understanding of immunoglobulin G (IgG) subclasses, their relative frequencies, and individual variations in the human population. IgG is the most abundant antibody isotype in the serum, providing long-term protection against infections.

**Why the Correct Answer is Right**
IgG1 is the most abundant IgG subclass, accounting for approximately 60-70% of the total IgG in the serum. It is produced in response to most protein antigens and has the highest affinity for the Fc receptor on phagocytic cells, making it the primary mediator of antibody-dependent cellular cytotoxicity (ADCC). The high individual variation in IgG1 levels is due to genetic differences in the immunoglobulin heavy chain locus, which influences the expression of the IgG1 heavy chain constant region.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG2 is the second most abundant IgG subclass, but it is primarily produced in response to polysaccharide antigens and has a lower affinity for the Fc receptor compared to IgG1.
**Option B:** IgG3 is the least abundant IgG subclass, but it has a higher affinity for the Fc receptor and is involved in the elimination of antibody-coated pathogens. However, its levels are not as variable as IgG1.
**Option D:** IgG4 is a functionally monomeric antibody that plays a role in the regulation of immune responses, but it is not the most abundant IgG subclass.

**Clinical Pearl / High-Yield Fact**
IgG1 is the primary mediator of ADCC, and its levels can influence the efficacy of antibody-based therapies, such as rituximab, which targets CD20+ B cells.

**Correct Answer: C. IgG1. The commonest Ig G with maximum individual variation is IgG1.**