If a date does not have an abbreviation after it, when do we assume this date
occurred?

after the year 1582 or simply after 1582

Explanation:

This is the case because it was in this year that the idea of writing dates without abreviations like B.C and A.D etc. was intraduced by Pope Gregory XIII in 1582.

For example, if we were to write this years date with abbrevation it would be 2020 C.E (the abbreviation indicates periods after Christ Era), that is, tweenty thousand and twenty years after Christ Era.  

i think your answer there is c