Choose the option that correctly fills the gap: By the time the rescue team arrived, the flood victims _______ for twelve hours.

The past perfect continuous tense 'had been waiting' is required here because the sentence describes an action that was ongoing over a period of time and was completed before another past event (the arrival of the rescue team). The phrase 'by the time' signals the need for the past perfect. Options A, C, and D do not correctly express this relationship between two past events.