Which of the following loads is a requirement to ensure proper transformer output?

To ensure proper transformer output, the ideal load configuration must be compatible with the transformer's design and intended usage. The dual-phase 120/240V load is particularly relevant because many transformers are designed specifically to serve residential and light commercial applications where both 120V and 240V circuits are common. This allows a transformer to effectively distribute power for various uses, including standard outlets, lighting, and larger appliances that require higher voltage.

In contrast, while single-phase 120V loads and single-phase 240V loads can be supported by transformers, they do not offer the versatility required to accommodate the broader range of household and commercial loads. Similarly, a three-phase 480V load, although suitable for industrial applications, does not align with the common residential wiring norms where the dual-phase configuration is more prevalent.

The dual-phase 120/240V load ensures that the transformer can deliver power effectively across different types of circuits, maximizing its utility in both residential and commercial settings. Thus, it is clear why this option is emphasized as a requirement for proper transformer output.