An appositive phrase renames the noun right next to it. In this case, the answer would be B, because the phrases 'triplet sisters,' and 'Laura, Lisa, and Lynn' are right next to each other without anything separating them.
'Lynn' isn't right next to 'triplet sisters,' so that wouldn't work, also the fact that 'Lynn' is only one of the sisters, so she can't represent all of them.
'All' and 'camp counselors' does represent the three, but they aren't side-by-side with the girls' names.
