Collaboration online increases students' motivation by
sparking an interest in a particular subject
introducing them to others with different viewpoints
holding students accountable to others
demanding that students contribute to a group

Only viable option remaining:  Holding students accountable to others.

Explanation:

Took the quiz twice, first time answered "introducing them to others with different view points", got it wrong.  2nd time answered "sparking an interest in a particular subject", got it wrong.

"Demanding that students contribute to a group" is not correct.

answer; ///i believe that the correct answer is (copper);