Hey there!
The purpose of an essay will be the main idea or point a writer is trying to make by writing the essay. Any tool that points out the main point or idea that they are trying to address will be a tool you can use to identify the purpose.
The only answer that applies based on that is your fourth option, "bracket sentences that address your point". The other three options might be good for identifying how different elements of the point were compared and contrasted in the essay, but the fourth option is the only one that has to do with the purpose directly.
Your answer will be your fourth option.
Hope this helped you out! :-)
