The correct answer is Option B.
The conclusion ends the paragraph by summing up the ideas expressed.
This is followed by the revision of the paragraph to look for any kind of error and ensure that the unity of the paragraph is maintained. This is the last step in paragraph writing.
Hence Option A and C are incorrect.
After the main idea, a writer has to illustrate upon it by explaining and providing suitable examples. Therefore Option B - illustration of the main idea is the correct answer.