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The correct answer is that the minimum ampacity required for the single-phase supply conductors of phase converters serving variable loads is 125% of the full-load amperes. This requirement is based on ensuring that the conductors can handle the increased current that may occur due to the starting and running characteristics of motors supplied by phase converters, particularly under variable load conditions.

When a phase converter operates, it often needs to accommodate the starting currents that are much higher than the normal running current. By specifying an ampacity of 125%, the National Electrical Code (NEC) aims to provide a safety margin that allows for typical variations in load and ensures that the conductors do not overheat, are capable of carrying the necessary currents, and can support proper circuit protection.

Choosing a lower percentage, such as 100%, fails to account for the transient conditions that can occur in variable loads, potentially leading to overheating and failure of the conductors. An even higher percentage like 150% may be unnecessarily large, leading to increased costs and inefficiencies. Therefore, 125% strikes a balance between safety and cost-effectiveness when dealing with variable loads served by phase converters.