Answer:
D. To explore an idea.
Step-by-step explanation:
An essay is any piece of writing that contains the author's argument(s). In other words, an essay is any long literary piece that discusses a single topic.
An essay can be of various topics, ranging from politics to domestic or international ideas. In general, essays are mainly about a single topic which the author expounds to make it more understandable, dissecting it into an easier and simpler explanation. We can also say that an essay is written to explore an idea, whatever the topic the author wants to or is expected to write about.
Thus, the correct answer is option D.