Final answer:
The topic sentence is important because it presents the main idea of the paragraph and each subsequent sentence should support this idea.
Step-by-step explanation:
The topic sentence is crucial in a paragraph as it introduces the main idea and guides the direction of the paragraph's content. The correct answer to the question is C. Each sentence in the paragraph needs to support the main idea in the topic sentence. This means that all subsequent sentences must provide evidence and examples to reinforce the topic sentence, ensuring the paragraph remains coherent and focused. The topic sentence, therefore, serves as a mini-thesis for the paragraph, tying back to the essay's overall thesis, and dictates the structure and development of the ensuing sentences.