Here is the item:
Le’Ron scowled at Professor Nguyen,
was muttering under his breath, and
sighed heavily, but in truth he loved taking algebra
exams.
When you list actions, you must use parallel structure. Here, scowled and sighed are both simple past tense verbs, but was muttering is past progressive, wrecking the parallelism.
What a delectable choice you made!
To fix the item, change was muttering to muttered.