Final answer:
The sentence with incorrect comma usage is C. Gretel always gets nervous, when she speaks in front of the class.
Step-by-step explanation:
The sentence with incorrect comma usage is C. Gretel always gets nervous, when she speaks in front of the class. In this sentence, there is a comma incorrectly placed after 'nervous'. The correct use of a comma in this context would be to place it after 'nervous' only if it is followed by a coordinating conjunction like 'but' or 'and'.
This error can be fixed by removing the comma, resulting in the sentence 'Gretel always gets nervous when she speaks in front of the class.'
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