Which sentence uses the words among/between correctly? a. our group divided the research tasks between the group members. b. the student leadership group was never really successful because they were always fighting among themselves. c. the fights among my sister and i were always really upsetting. d. the sense of competition among the two runners was palpable.

This is the correct  B. The student leadership group was never really successful because they were always fighting among themselves.

The difference in using among vs. between is that generally between is used for specific items and individuals, whereas among is used for things that are not distinct items or when talking about a group of people. A, is talking about a group of people so it should be among. C, is talking about two specific people, so it should be between. D, is talking about two specific people so it should be between. Therefore, B is the correct answer.