the purpose of a letter to an editor is\nto persuade.\nto entertain.\nto inform.\nto teach.

Brief Explanations:

A letter to an editor is typically written to express an opinion on a particular issue and encourage the editor (and by extension, the readers) to consider that viewpoint. This is the essence of persuasion. It's not mainly to entertain (like a story), just to inform (a news article might focus more on pure information delivery without the opinion - based push), or to teach (a how - to guide or educational text).

Answer:

to persuade.