The correct answer is: B. will have been dieting/will have lost
The sentence is in the future perfect continuous tense, which is used to talk about an action that will be in progress at a specific time in the future. In this case, the specific time is when the person leaves for their holiday. The action is “dieting” and the result of the action is “losing weight.”
The other options are incorrect because they do not use the future perfect continuous tense. Option A, “have been dieting/will be losing,” is in the present perfect continuous tense. This tense is used to talk about an action that started in the past and is still in progress. In this case, it would be used to talk about someone who has been dieting for some time and is still dieting at the time of the holiday. Option C, “had dieted/lost,” is in the past perfect tense. This tense is used to talk about an action that was completed before a specific time in the past. In this case, it would be used to talk about someone who had dieted in the past and had lost weight before the holiday. Option D, “were dieting/had lost,” is in the past progressive tense. This tense is used to talk about an action that was in progress at a specific time in the past. In this case, it would be used to talk about someone who was dieting at a specific time in the past and had lost weight. Option E, “are going to diet/have lost,” is in the future tense. This tense is used to talk about an action that will happen in the future. In this case, it would be used to talk about someone who is going to diet in the future and will lose weight.
To summarize, the correct answer is B because it uses the future perfect continuous tense, which is the only tense that is appropriate for this sentence.