The answer is I, II, and III.
It violates society´s ethical code because plagiarism is a form of fraud or theft which is considered, in any of their forms, an ignoble and unedifying action according to society´s standards.
By passing somebody else´s words as your own, you are preventing the author of the words to receive due and earned credit, credit that, for the author of the words (which make up ideas, do notice that) may have involved dedicated, strenuous and considerable efforts in conforming.
Finally, it is dishonest and damages your own personal integrity for the reasons stated above and because it diminishes the trust that society can bestow upon you, damaging your reputation and tainting your name.