Final answer:
Option C, 'Committed people are also often stubborn,' would not be a good supporting sentence for the topic sentence.
Step-by-step explanation:
Based on the given topic sentence, 'I have never seen a person so committed to a cause,' the supporting sentence that would not be a good fit for an illustration paragraph is option C. 'Committed people are also often stubborn.' This sentence does not provide a reason for the topic sentence's claim, an illustration of the topic sentence's point, an explanation of the sentence's point, or evidence demonstrating the topic sentence's point. It is a general statement that does not contribute to the understanding of the main idea.