Concept:Began means started. The opposite idea is to stop or come to an end, which is
cease.
Solution:The sentence contrasts the start of something ("when this began") with its end.
So the gap needs a word meaning "end": "It is hard to tell when this began or where it will cease."
A. "fall" means to drop down, not to end.
C. "reside" means to live in a place.
D. "remain" means to stay or continue, which is close to the opposite of ending.
Key point: begin / cease (or begin / end) are opposites; "remain" is a trap because it is about continuing.
Answer:Option B (cease)