A) is not read closely by many audiences
When you wirte an essay or a paper for school, the first thing that you have to bear in mind is your audience. To whom will be directed? Who will read it? Once clarified that, you can start writing. It is not the same to write for scientists that for your peers at school. Everyone will read it diferently.
Regarding the other options, once you consider your target, you should know if the register used will be formal, or informal.
Everything you write should be considered important, regardless of the topic. To be a good writer, you have to write from the heart, and that is not fulfilled if you do not consider it important.
