The correct answer is B. has been planning.
The sentence is in the present perfect continuous tense, which is used to talk about an action that started in the past and is still happening now. In this case, Monica has been planning something since last week, and she is still planning it now.
The other options are incorrect because they do not express the continuous aspect of the action. Option A, did plan, is in the simple past tense, which is used to talk about an action that happened at a specific time in the past. Option C, had been planning, is in the past perfect continuous tense, which is used to talk about an action that started and ended in the past. Option D, was planning, is in the past progressive tense, which is used to talk about an action that was happening at a specific time in the past.
Here are some examples of how the present perfect continuous tense can be used:
- I have been working on this project for two weeks.
- They have been waiting for us for an hour.
- The train has been delayed for two hours.
- The baby has been crying for ten minutes.