The correct answer is: B. have been preparing/haven’t bought
The sentence is about someone who has been preparing for a surprise party all week, but has not yet bought a birthday present for the guest of honor. This suggests that the person has been busy with the preparations and has not had time to buy a present. The verb “prepare” is in the present perfect continuous tense, which indicates that the action of preparing has been going on for some time and is still ongoing. The verb “buy” is in the simple present tense, which indicates that the action of buying has not yet happened.
The other options are incorrect because they do not accurately reflect the meaning of the sentence. Option A, “are preparing/didn’t buy,” suggests that the person has been preparing for the party and has already bought a present. Option C, “prepared/don’t buy,” suggests that the person has prepared for the party but has not yet bought a present. Option D, “have prepared/aren’t buying,” suggests that the person has prepared for the party and is not going to buy a present. Option E, “prepare/weren’t buying,” suggests that the person is preparing for the party and was not going to buy a present.