Firstly, there must be a strong topic sentence. If the paragraph is in an essay, the topic sentence must be connected to the thesis, supporting a claim, and must give an idea of the purpose and content of the following paragraph. If the paragraph is stand-alone, then the topic sentence must be a claim or stance on the prompt or thought process backed with reasons that can later be explained through evidence as the paragraph develops.
Evidence is a very important factor in a paragraph stand-alone or in essay form. There must be adequate evidence such as a quote, or a line, or research with cited sources, that support the claim that was made in the topic sentence. This evidence proves that you know what you are talking about and that you have clear and educated reasoning.
Grammar. If you are writing a formal paragraph, avoid contractions (i.e. can't) and avoid personal pronouns (i.e. I, you, we), only using he she it and they. Also, be sure to not start sentences with "and" or "because", and avoid phrases such as "this proves my point because..." and "I believe that this is correct because.", or "this evidence shows...". You must be decisive, efficient and concise.
Closing. Once adequately backing your claims with evidence and proper reasoning, you need to close the paragraph. If it is an essay, close the paragraph by connecting the entire paragraph's message back to the thesis and include some sort of transition that leads to the next paragraph. If it is a stand-alone paragraph, then you must connect the closing sentence or two back to your initial claim without being obvious and without repeating words. Then, find a unique way to eloquently end the paragraph with your final words.
An example of a closing for Ayn Rand's Anthem, is:
"The World Council suppressed him into the dust and ashes of shame, misery, and suggested communism, but Equality refuses to conform, and bursts into a flaming individual, who rises from the ash of where he is defeated, because nothing, not anything at all, can hinder the light that shines from within."
answer: c. like someone who knows how to behave in public