According to NFPA 70, an insulated conductor used within a switchboard or switchgear shall be rated not less than the voltage applied to it and:

According to NFPA 70 (National Electrical Code), Section 408.19, the insulation on a conductor within a switchboard or switchgear must be rated for two conditions. First, it must be rated for the voltage applied to that specific conductor. Second, it must also be rated for the highest voltage of any other conductor or busbar it might physically touch. This requirement prevents insulation failure and potential arc faults caused by contact between conductors of different circuits or voltage levels that are often routed in close proximity within the confined space of electrical equipment.

A. the voltage applied to the equipment ground: The equipment ground has a nominal potential of zero volts, so this rating would be insufficient and irrelevant for insulation purposes.
C. 600 volts, regardless of the applied voltage: While 600V is a common insulation rating, it is not the universal requirement. The rule is relative to the actual voltages present, which could be higher or lower.
D. the highest phase-to-phase voltage of the system: This is less precise. The conductor only needs to be rated for the voltage of components it may contact, not necessarily the highest system voltage if it is segregated from those components.

1. NFPA 70, National Electrical Code, 2023 Edition.

Article 408, Section 408.19, "Conductor Insulation." The text states: "An insulated conductor used within a switchboard or switchgear shall be listed, shall be flame retardant, and shall be rated not less than the voltage applied to it and not less than the voltage applied to other conductors or busbars with which it may be in contact."

2. NFPA 70, National Electrical Code Handbook, 2023 Edition.

Chapter 4, Article 408, Exhibit 408.5 (Commentary on 408.19). The handbook explains the rationale: "The requirement for conductor insulation to be rated for the voltage of other conductors or busbars with which it may come in contact is necessary because conductors of different systems are permitted to be installed in the same switchboard or switchgear." This commentary confirms the purpose is to prevent dielectric breakdown between adjacent conductors of different voltage levels.