Final answer:
The key part of a paragraph is the topic sentence. It provides a clear and concise statement that introduces the main point that will be discussed in the paragraph.
Step-by-step explanation:
The key part of a paragraph is the topic sentence. The topic sentence is the main idea or focus of the paragraph and is usually found at the beginning of the paragraph. It provides a clear and concise statement that introduces the main point that will be discussed in the paragraph. It helps to organize and unify the paragraph by guiding the reader and establishing a clear purpose.