The true intent of a paragraph is to express coherent points. The main idea of a paragraph always flows in one direction. As soon as this idea changes, the paragraph also changes.
Hence, a paragraph has one idea and everything familiar that naturally flows with it fits into one single paragraph. Thus, first is unity.
Secondly, a paragraph must follow an order. As ideas in a paragraph should not be random, They should be arranged into a logical chronological order that flows in one direction making it easier to read.
Thirdly, the main idea of the paragraph should not be hanging loose. There should be coherence to interpret the main idea.
Lastly, the paragraph ought be complete in itself.
Therefore, B is correct.
A is incorrect as topic and length are very loose basis to form a paragraph on.
C is incorrect as information can be random also.
D is incorrect as conclusion always comes at last. Hence, rendering this option incorrect.