The correct answer is D. Shall leave.
The future tense is used to talk about things that will happen in the future. There are three main ways to form the future tense in English:
- Simple future: I will leave.
- Present continuous: I am leaving.
- Future continuous: I will be leaving.
The simple future is used to talk about things that will happen in the future with certainty. The present continuous is used to talk about things that are happening now and will continue to happen in the future. The future continuous is used to talk about things that will be happening in the future at a specific time.
In this case, we are talking about something that will happen in the future with certainty, so we should use the simple future tense. The correct answer is D. Shall leave.
The other options are incorrect because they are not in the simple future tense. Option A, Leave, is in the simple present tense. Option B, Shall leaving, is not a grammatically correct sentence. Option C, Am going to leaving, is in the present continuous tense.
I hope this explanation is helpful. Please let me know if you have any other questions.