The correct answer is B. You said that they were busy the whole day.
The sentence “They were busy the whole day” is in the simple past tense. This tense is used to talk about actions that happened at a specific time in the past. In this case, the speaker is saying that the people they are talking about were busy at a specific time in the past, which is the whole day.
The other options are incorrect because they use different tenses. Option A uses the past perfect tense, which is used to talk about actions that happened before another action in the past. Option C uses the past perfect progressive tense, which is used to talk about actions that were happening before another action in the past. Option D uses the present perfect tense, which is used to talk about actions that started in the past and continue to the present.
In this case, the speaker is not talking about actions that happened before another action in the past, or actions that were happening before another action in the past, or actions that started in the past and continue to the present. The speaker is simply talking about actions that happened at a specific time in the past, which is the whole day. Therefore, the simple past tense is the correct tense to use.