How does the searching vessel's radar interrogate a survival craft SART?

The correct answer is based on the operational characteristics of the Search and Rescue Transponder (SART). When a SART, which is designed to assist in locating survival crafts, is within range of a searching vessel's radar, it automatically detects the radar signal operating in the X-Band frequency range (typically around 9.3 GHz).

Upon detecting this radar signal, the SART activates and sends back a series of 12 blips or pings on the radar screen of the searching vessel. This distinct signal pattern helps in the visual identification of the SART's location on the radar display, making it easier for search and rescue operations to pinpoint the survival craft.

The other options describe elements that do not accurately reflect how a SART operates. For example, the IFF interrogation system is related to military operations and not applicable to SART function. Additionally, while radar signals are involved, SART specifically responds to the X-Band frequency, so references to other radar signals or watchkeeping on VHF channels do not apply in this context. The key fact is that the correct functioning of a SART is contingent on its interaction with X-Band radar, which is fundamental to its purpose in search and rescue scenarios.