Which vessel shall operate white flashing lights flashing the Morse letter 'U'?

The vessel that operates white flashing lights identifying the Morse letter 'U' is an oil rig when stationary and engaged in drilling or production. In maritime signaling, specific light patterns and colors indicate the activities and characteristics of vessels involved in operations. The Morse code 'U', represented by two short flashes followed by a long flash, is designated for offshore drilling units. This signaling helps other vessels recognize the rig's operational status and take appropriate navigational actions to avoid collision or unsafe interactions.

The other options display different operational contexts that do not align with this particular lighting signal. For example, cargo ships at port have their own unique lighting regulations that indicate their moored status. Fishing vessels anchored might use lights that signify certain forms of fishing activity rather than indicating an oil rig's operational state. Likewise, passenger ferries in transit would have a different set of recognizable signals tailored to their specific passenger transport function. Thus, while all vessels follow the regulatory framework of COLREGs, only the stationary oil rig uses the specific white flashing lights that convey the Morse letter 'U'.