Question

They could not decide which one they like and ..... the end they didn't brother​

Answer 1

Explanation:

by the time when the end came they didn't bother

by

alone would also work.

The other responder used in. I hadn't thought of that. Common usage would probably use "in" but what I have pointed out as a use is also possible. The point is that whatever you use, it must be a preposition.

Answer 2

Answer:
in

Explanation:
By could also be used if there is an extensively long series of events before that “end” occurs. Other than that, “in” is most commonly written.