Hello there! I would go with answer D. an exploration of ideas!
This would be the best option especially because the type of essay isn't specified. A would define a persuasive essay; and B and C would define an informative essay. As I said earlier, the type of essay is not specified in the question, so D would be you answer since it is also a valid definition.
I hope this helps & have a great rest of your day! :)
