The answer is Why will I be writing?
Explanation:
One of the most important elements in writing is the purpose of writing or why the writer writes. This element influences the claim or thesis statement in the text. For example, if the purpose is to persuade, the claim should support a specific point of view.
Moreover, the purpose is related to the prompt (instructions for writing) because by analyzing the purpose (persuade, explain, describe, etc), it is possible to understand how to write a text according to the prompt. In this context, why will I be writing? is the question that helps understand the prompt and develop a claim.