Question

Which accurately describes a letter written during the time a historical event occurred?

Answer 1

A historical document ? Sorry, I think your question is incomplete or unclear to me haha. But if you need on the other hand to recognize an especific historical document then you must check for dates, language (as in grammar and vocabulary to check corespondance to other times as language is constantly actualized) and names of places and people (as places have a date of creation and people have birth certificates or similar registrations)

Answer 2

I think maybe there were some options that you didn't copy?

Else I can say that a letter written at the time of a historical event would be a primary source - as opposed to a secondary source which is written well after the relevant event.