Which high side rating indicates a transformer is suitable for a delta connection only?

The high side rating that indicates a transformer is suitable for a delta connection only is 12000-. In a delta connection, the transformer must be designed to handle line-to-line voltage, which is typically represented by these higher voltage ratings. The designation of 12000- means it can handle 12,000 volts line-to-line.

In contrast, the other options indicate either lower voltage ratings or specific configurations that are not exclusively delta. For example, options like 240/120 or 120/240 represent transformers designed for residential or commercial applications, typically used in wye (Y) configurations for lower voltages, where the line-to-neutral voltage would be more relevant.

Therefore, the correct answer reflects the characteristics of a transformer that is solely intended for use in delta configurations and is rated accordingly to manage the higher voltages found in such systems.