What were the five principles agreed to at potsdam conference?

Allied leaders met to discuss a postwar settlement and they agreed on five principles
1. Germany remains single country
2. Germany must be demilitarized
3. Outlaw Nazi Party
4. Rebuild German political structure
5. Anyone responsible for war crimes should be trialed.

the fourth alternative is correct (d).

all of these alternatives can be used. when we are talking about something that happened in the past, it is not easy to remember precisely the date it occurred. thus, we can use the beginning / end of a century or decade or some striking event that has occurred close to and is of wide knowledge. this is a way to reference the time for the interlocutor to contextualize it.