An arbiter is consulted when
a. neither side can agree to move forward
b. both groups agree on a compromise
c. a cooperative decision is made
d. someone has their feelings hurt

If you look at the options b) and c) - they are very similar, almost the same! So it is unlikely that only one of them would be correct, either both or none ,so it is none!

An arbiter is asked for help when people can't reach a compromise without him or her - so the correct answer is a)!