The correct answer is: has been.
The sentence is in the past progressive tense, which is used to describe an action that was in progress at a specific time in the past. In this case, the specific time is “when you rang me up”. The action that was in progress at that time was “raining”. Therefore, the correct verb tense to use is the past progressive tense, which is formed with the verb “to be” in the past tense and the present participle of the main verb. In this case, the present participle of the verb “to rain” is “raining”. Therefore, the correct answer is “has been raining”.
The other options are incorrect because they do not use the past progressive tense. Option A, “had been raining”, is in the past perfect progressive tense, which is used to describe an action that was in progress before another action in the past. Option B, “has been”, is in the simple present tense, which is used to describe an action that is happening now or that happens regularly. Option C, “have been”, is in the simple present plural tense, which is used to describe an action that is happening now by more than one person.