To make sure your reader remembers the point of your essay, which of the following is most important?
a. the order in which you present your ideas
b. choosing an interesting subject
c. the introduction
d. the ending

Well, to be honest, I don't think there is an absolute answer - but i'd go for A. Let's see them one by one:

A. The order in which you present your ideas
-IF your idea is an argument, then it's very important that one thing follows from another- and for this the order is important. If the reader understands your point, then they will remember it well: this is well known: we remember the things we've "understood" better. ,. If they don't- they won't.

B. Choosing an interesting subject - no, i think this is in any case not a good option

C. The introduction - if your reader can choose whether or not to read your work, then this might very well be the decisive point, (they simply won't read it if this part is bad!) but let's assume that they have to read it. then it's not as important as the order.

D. The ending - well, there is also the recency effect: we remember best what came at the end. So if you don't have an argument, but let's say, the claim - this might be more relevant.

In general, I would go for A, but I think it depends on the text