Concept:The blank needs a word that contrasts with “began” and describes the point where something stops or ends.
Explanation:The sentence tells us that the starting time of an event is difficult to identify, and so is the place where it stopped.
The word “ceased” means to stop, discontinue, or come to an end.
It forms a clear contrast with “began,” completing the idea: “It is hard to tell when it began or where it ceased.”
This means the speaker cannot say when the action started, nor can they say where it came to an end.
The other options are not suitable because “remained” and “resided” both relate to staying or living somewhere, not to ending.
“Felled” means to cut down, such as a tree, which does not fit the sense of the sentence at all.
So the correct choice is the only option that gives the opposite meaning of “began” and fits grammatically and logically in the sentence.
Answer:C. ceased