The correct answer is: A. had known
The sentence is in the past perfect conditional, which is used to talk about something that would have happened in the past if something else had happened. The past perfect is formed with the past participle of the verb (known) and the auxiliary verb had.
In this sentence, the speaker is saying that they would have waited for the listener at the station if they had known that the listener would come. The speaker did not know that the listener would come, so they did not wait.
The other options are incorrect because they do not use the past perfect tense.
- B. was knowing is in the past progressive tense, which is used to talk about something that was happening at a specific time in the past. In this sentence, the speaker is not talking about something that was happening at a specific time in the past, they are talking about something that would have happened if something else had happened.
- C. have known is in the present perfect tense, which is used to talk about something that has happened or been true up to the present moment. In this sentence, the speaker is not talking about something that has happened or been true up to the present moment, they are talking about something that would have happened if something else had happened.
- D. No improvement is incorrect because the sentence is already correct.