As with any successful essay, the question must be understood before an effective answer can be given. With a question that uses an "extent" in its stem, a case with evidence and analysis must be made. The idea of "To What Extent" is asking that the answer discuss how one element is greater in validity than others. The "To What Extent" question is asking that the student is able to make a claim about an assumption in a question and then place it in context and assess other conditions that surround the assumption. In the writing process, the student is making a definite claim whether it is to a "great extent" or "not a significant extent" that the assumption in the question is valid or verifiable. It requires the student to understand a specific context as well as the different conditions surrounding it in order to make a call to validity.
In answering a question that uses "To What Extent" as its stem, I think that the first part of the essay should focus on the assumption in the question. This will required support, elaboration and substantiation in making a particular case if something is valid to a great extent or not. Analysis and evidence would be critical here. The second part of the essay should focus on the role that other factors play in the question's assumption. Any question that uses "To What Extent" has to pull a topic that has other factors at play. For example, in the question, "To what extent can nationalism be seen as the primary cause of World War I," the understanding is that there were other causes to World War I and the critical element is to assert if Nationalism was the primary cause and how other causes would or would not be serve as the primary cause. The last part of the essay should reaffirm what the analysis and evidence presented has (hopefully) concluded. Put another way, is it to a great extent or not large extent that the assumption of the question can be seen? In crafting an essay with the "To What Extent" prompt, I think that this method becomes the most direct.