A thesis statement should focus on your ideas and expose it well in one or two sentences. It should present the topic of your paper and also make a comment about where you stand on it. A question of fact proposes that something is a fact. Similarly, a question of value proposes the value of something, usually in comparison with something else. Neither of those are the case here.
"A purpose statement is a declarative sentence which summarizes the specific topic and goals of a document"
That is what happens in this question and that's why the answer is:
b.
purpose statement
