Which raceway type is not allowed for the protection of a grounding electrode conductor smaller than 6 AWG?

The correct answer is LFNC (Liquid-Filled Non-Metallic Conduit) because the National Electrical Code (NEC) has specific regulations regarding the types of raceways that can be used for certain applications, particularly when it comes to grounding and bonding conductors. According to the NEC, LFNC is not permitted for the protection of grounding electrode conductors that are smaller than 6 AWG due to its limitations in providing adequate mechanical protection and grounding capabilities.

In contrast, raceway types like rigid conduit, flexible conduit, and non-metallic conduit have been recognized and can be used in various applications, including with grounding conductors, as long as they comply with the NEC's standards for grounding electrode conductors. Rigid conduit provides strong physical protection, flexible conduit offers some flexibility while still being adequate for protection, and non-metallic conduit can be used if it meets the requirements for corrosion resistance and suitability for the type of installation. Thus, LFNC stands out as the only option that does not meet the necessary criteria for this purpose.