Which one expresses the main idea of a paragraph -

Topic sentence expresses the main idea of a paragraph. 

- A paragraph is a series of sentences that are organized and coherent, and are all related to a single topic.
- Almost every piece of writing you do that is longer than a few sentences should be organized into paragraphs.
- One of the most important of these is a topic sentence.

- Topic Sentence: A well-organized paragraph supports or develops a single controlling idea, which is expressed in a sentence called the topic sentence. A topic sentence has several important functions: it substantiates or supports an essay’s thesis statement; it unifies the content of a paragraph and directs the order of the sentences; and it advises the reader of the subject to be discussed and how the paragraph will discuss it. Readers generally look to the first few sentences in a paragraph to determine the subject and perspective of the paragraph. That’s why it’s often best to put the topic sentence at the very beginning of the paragraph. 

Reference: Indiana University [লিঙ্ক]