Here is the item:
Ernie is too noisy as a seatmate. Crunching on apple
slices, slurping hot coffee, and
the rings of his notebook snapped open and shut
all contribute to the cacophony.
Good answer! When you list grammatical items, you must use parallel structure. Here, crunching on apple slices and slurping hot coffee are both gerund phrases, but the rings of his notebook snapped open and shut, a main clause, ruins the parallelism of the list.
To fix the item, change the rings of his notebook snapped open and shut to snapping the rings of his notebook.