Final answer:
The complete verb in the given sentence is 'had been promised'. The past perfect tense is used to indicate the promise was made before a specific time in the past, and the passive voice is used to show that Rebekah was the recipient of the promise.
Step-by-step explanation:
The complete verb phrase in the given sentence is had been promised. In this sentence, the action of being promised two sons by the Lord is expressed using the verb phrase had been promised. The past perfect tense (had been) is used to indicate that the promise was made before a specific time in the past. The passive voice is used to show that Rebekah was the recipient of the promise.