Review the item:
At the back of the classroom, Nina sat with her arms crossed, glaring at
her teacher, Professor Beane,
her body language indicated that English literature was her least
favorite subject.
For a sentence to be fused, you must have two complete sentences run together with no punctuation. In the example above, a comma joins the two sentences. Nina sat with her arms crossed ... begins one sentence. Her body language indicated ... starts the second sentence.
Thus, this error is a comma splice.
To fix the problem above, you could put a period after Beane and begin her with a capital H.