Answer:
I would say that it would be best to delay stating the main idea until after all the supporting details have been presented when the topic of the essay will be new or unfamiliar to most readers. The correct answer would be the last one.
Step-by-step explanation:
I chose the last one because, this being the case, it would be a good choice for the author to first present the subject and explain its details that are totally new for the audience, and then, at the end, make the statement, like if it was a kind of conclusion of everything he explained.
This, for me, would only be possible if we, as authors, know that our audience is not aware of the facts of the subject, and we first need to make a kind of big introducion to make our points. If not, this wouldn't be possible because in a regular essay we put the statement first and then we discuss the details to support the statement; we don't keep our opinion to ourselves to create suspense in an essay, it is just not that kind of text. We need to be more formal in an essay.