Per Table 250.66, a service with 2 AWG copper (or smaller) ungrounded conductors requires a copper grounding electrode conductor (GEC) of at least what size?
Not quite. The answer is D: 8 AWG
Table 250.66 sets 8 AWG copper as the minimum grounding electrode conductor when the largest ungrounded service conductor is 2 AWG copper or smaller. The GEC grows with the service conductors, up to 3/0 AWG copper for the largest services. Separately, 250.66 limits the GEC to 6 AWG copper when its only connection is to a rod, pipe or plate electrode, and to 4 AWG copper when its only connection is to a concrete-encased electrode.
Reference: NEC 2023 Table 250.66 / 250.66 (Grounding Electrode Conductor Sizing)
Checked against multiple published sources
An NFPA fact sheet (2020 NEC, June 2020) reproduces the Table 250.66 row for 2 AWG copper or smaller (8 AWG copper) and the 6 AWG / 4 AWG caps in 250.66(A) and (B); an IAEI article agrees on the 8 AWG minimum and the caps. The fact sheet is NFPA's summary, not the code text, and is written against the 2020 edition.