An 'epistle' is commonly defined as:

An 'epistle' is defined as a long, formal letter, often written to convey important messages or teachings. This term is frequently associated with literary or religious contexts, especially in reference to the letters found in the New Testament of the Bible. In these contexts, epistles serve as means of communication that provide guidance or instruction to the recipients. The formal nature of an epistle distinguishes it from everyday correspondence or casual notes.

Considering the other choices, a fictional story does not align with the definition of an epistle, as it refers to a narrative work rather than a form of communication. A scientific report, while formal, is specifically focused on presenting research findings, which is different from the personal or instructional intent of an epistle. An informal note lacks the structure and seriousness typically associated with epistles, thereby further differentiating it from the correct definition.