Take a look at the item again:
Mrs. Neal loves chocolate cake. (A)
But couldn't accept the slice that her student Michael
offered (B)
because she knew that it was a bribe
(C) for a better grade
on his midterm essay.
You wanted to do this:
Mrs. Neal loves chocolate cake. But couldn't accept the slice that her student
Michael offered because she knew that it was a
bribe, for a better grade on his midterm essay.
A comma after bribe is an error. For a better grade on his midterm essay is a string of prepositional phrases. When you attach such phrases at the end of a sentence, you do not need punctuation.
You might want to review the rules.
Go back to the sentence to try again.