What transformer rating indicates that it is good for a wye connection only?

The rating that indicates a transformer is suitable for a wye connection only is characterized by the presence of a ground symbol in the configuration labeling. In this case, the choice that is correct shows a grounded wye connection with the "GrdY" notation, which signifies that the system is both a wye configuration and has a grounded neutral.

Using a grounded wye configuration improves safety and stability, especially in providing a low-impedance path to ground, which helps mitigate line-to-ground fault conditions. This design is crucial for supplying single-phase loads from a three-phase source while maintaining overall system integrity.

In contrast, the other options either do not include the grounded notation or suggest configurations that are not exclusively for wye connections. For example, options with just 'Y' or without a grounded reference indicate that they might support other configurations, such as delta or ungrounded wye, which has distinct electrical and operational implications.