What statement should you use if you do not recall the exact date of an event?
a. near the end of the 20th
c. around the time of
b. in the late 1900’
d. any of these would work

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.

the awenser wuold be slaves

nothing certain is known about what happened, but it seems that the country declined gradually, perhaps due to bad government, or, more likely, due to a series of bad harvests, which resulted in widespread and recurrent famine.