**Core Concept**
In one-carbon metabolism, serine is converted to glycine via the enzyme serine hydroxymethyltransferase (SHMT). This reaction is a key step in the transfer of one-carbon units, which are essential for the synthesis of nucleic acids, amino acids, and other biomolecules. The conversion of serine to glycine involves the transfer of a carbon atom from serine to the coenzyme tetrahydrofolate (THFA).

**Why the Correct Answer is Right**
The correct answer involves the identification of the carbon atom that is added to THFA during the conversion of serine to glycine. In this reaction, the carbon atom from serine is transferred to the N5 position of THFA, forming 5,10-methylenetetrahydrofolate (5,10-CH2-THFA). This is a crucial step in the one-carbon metabolism pathway, allowing for the subsequent transfer of carbon units to other molecules.

*Option A:* Incorrect - The carbon atom from serine is not added to the N10 position of THFA.
*Option B:* Incorrect - The carbon atom from serine is not added to the C6 position of THFA.
*Option C:* Incorrect - The carbon atom from serine is not added to the C9 position of THFA.

**Clinical Pearl / High-Yield Fact**
One-carbon metabolism is a critical pathway for the synthesis of nucleic acids, and defects in this pathway can lead to disorders such as homocystinuria and megaloblastic anemia.

**Correct Answer:** C. The carbon atom from serine is added to the C6 position of THFA.