An ellipsis is primarily used as a literary device in the following ways:
B. to shift time in a story - An ellipsis can indicate a passage of time or a jump in narrative, allowing the author to skip ahead or to indicate that certain events have taken place without detailing them.
C. to pause a story - It can create a pause for dramatic effect, allowing the reader to absorb the weight of the moment or what is being said.
Based on these uses, both B and C are correct in the context of how an ellipsis functions in literature.
However, if you must choose one option specifically related to shifting narrative time, the best answer would be:
B. to shift time in a story.