Plagiarism is the use of another author work, ideas or words without crediting it, it means when you plagiarize you are presenting the ideas or work of someone else as if were yours which is morally incorrect and dishonest and also led to serious sanctions as it is considered as copyright infringement. Considering this is dishonest and has negative consequences, it is important to avoid plagiarism every time you write a document or present some work, for doing this you will need to make sure most of the work is yours and you just use other authors to support your work, also, if you know you are using someone else work or ideas it is important to credit the source which means when you quote (take the words of someone) or paraphrase (express the words of someone with your own words) you tell the reader from who is the author of those ideas and words and in which resource (book, article, web page, etc) you took those words. This is important as just paraphrasing is not enough because you will still have to recognize those ideas are not yours. Thus, one way to avoid plagiarism is B. II only (Identifying quotations and crediting the source properly).
Thesis: a statement or theory that is put forward as a premise to be maintained or proved.
Supporting your thesis is the overall goal of your whole paper. It means presenting information that will convince your readers that your thesis makes sense. You need to take care to choose the best supporting details for your thesis.