Final answer:
The troposphere is the layer closest to the Earth's surface and one of the most critical layers in the Earth's atmosphere.
Step-by-step explanation:
The best topic sentence for this paragraph is C. The troposphere is the layer closest to the Earth's surface. This topic sentence clearly states the main idea of the paragraph and provides information about the specific layer being discussed. It sets the focus for the rest of the paragraph, which goes on to explain the importance of the troposphere and its characteristics in more detail.
Learn more about The topic of this question and answer is the layers of the Earth's atmosphere