The correct answer is A. knew.
The sentence is in the past conditional tense, which is used to talk about hypothetical situations that did not happen. In this case, the speaker is saying that if they had known the person’s number, they would have phoned them. The past simple tense (knew) is used to talk about events that happened in the past. The present tense (know) and the present perfect tense (has known) are not used in this context because they are used to talk about events that are happening now or that have happened in the past and continue to happen in the present. The present perfect continuous tense (has been knowing) is not used in this context because it is used to talk about events that have been happening for a period of time and are still happening.
Here are some examples of how the past conditional tense is used:
- If I had known you were coming, I would have cooked dinner.
- If I had studied harder, I would have passed the test.
- If I had been there, I would have helped you.
I hope this helps! Let me know if you have any other questions.