The answer is: B. It contains one single idea.
Unity for a paragraph implies that every paragraph consists of one single idea, which is developed in the topic sentence. As a consequence, the rest of the sentences in each paragraph must provide evidence to support the subject that the topic sentence expresses.
The rest of the options are incorrect because they do not express unity or the thought that a paragraph is seen as a whole for itself. A paragraph does not necessarily have to be simple, brief and focused. However, paragraphs must contain evidence and the sentences should be connected so that they are easier to read.