**Core Concept**
The capacity to produce immunoglobulin G (IgG) is a significant milestone in the development of the immune system in humans. IgG is the most abundant antibody isotype in the body and plays a crucial role in providing long-term immunity against infections.

**Why the Correct Answer is Right**
The production of IgG starts at approximately 6 months of age, when the gut-associated lymphoid tissue (GALT) and the spleen mature enough to support the production of this antibody isotype. This coincides with the introduction of solid foods and increased exposure to antigens, which stimulates the immune system to produce IgG antibodies. The production of IgG is mediated by activated B cells, which differentiate into plasma cells that secrete IgG antibodies.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the production of IgG does not start at birth. While IgG antibodies are present in the newborn from the mother's circulation, the infant's own immune system does not produce IgG until 6 months of age.

**Option B:** This option is incorrect because the production of IgG is not a function of the thymus. The thymus is responsible for the maturation of T lymphocytes, which are involved in cell-mediated immunity, not antibody production.

**Option C:** This option is incorrect because the production of IgG is not a function of the liver. While the liver produces many proteins, including some antibodies, the production of IgG is a function of the immune system, specifically activated B cells in the spleen and lymph nodes.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the production of IgG is a critical milestone in the development of the immune system, and its absence or deficiency can lead to increased susceptibility to infections.

**Correct Answer:** C.