Final answer:
The grammatically correct option to replace (20) in the passage is 'B. Been', as it completes the past perfect tense 'had been' necessary for the context.
Step-by-step explanation:
The correct option to maintain grammatical accuracy in the passage replacing (20) is 'had been'. The passage is discussing an event in the past that was complete before another event; this is often conveyed by the past perfect tense, which is formed with 'had' + the past participle of the verb. Therefore, the correct option would be 'B. Been', as it would correctly complete the past perfect tense 'had been' in the sentence.
The Venetians 'had been' away so long and were so much changed in appearance that none of their relations and old friends knew them when they arrived in Venice. The use of the past perfect tense here is needed to indicate that their long absence was complete by the time they returned to Venice.