C  
Ion-exchange high-performance liquid chromatography separates HbA1c from other hemoglobin fractions based on charge differences, offers high precision, and is the reference method used by NGSP/IFCC for standardization, making it the preferred (“gold-standard”) assay for HbA1c measurement [Weykamp 2009, PMID 20144280; Lahousen 2002, PMID 12212363].