The correct answer is Option B
The question is on the salient or most important feature of a strong paragraph.
The length of a paragraph in no way contributes to its strength. A paragraph should be comprehensive. Therefore, Option A is the incorrect option.
A paragraph can be written on any topic. It is thus not a feature of a paragraph - making option C the incorrect option.
Similarly, the writer's vocabulary does not really contribute to the making of a strong paragraph if the writing is not comprehensive or the idea expressed in the paragraph is not unified. This makes option D incorrect.
A strong paragraph contains one main idea supported by supportive ideas. This unity is an essential feature of paragraph writing. Hence option B is the correct answer.