Which best explains why frankenstein is referred to as an epistolary novel?
it is told, mostly, in the first person.
it uses gothic elements.
it is told, in part, through letters.
it uses a frame structure.

It is told,in part, through letters

Epistolary means a literature written using  letters. Frankenstein is also a novel  written in the form of letters. Mary Shelly is the author of the novel and the author feels that the epistolary form of writing makes the reader, read the story with complete interest and concentration.

The narration of the novel is in a very confusing way. The epistolary type  of writings makes the readers feel the actual context of the story. It makes the readers feel as if the characters are writing to them.