Final answer:
All the provided options constitute plagiarism, which is unethical and involves taking someone else's work or ideas without proper credit.
Step-by-step explanation:
Plagiarism is the act of presenting someone else's work or ideas as your own without proper attribution. When looking at the options given in your question, all the described actions – failing to cite sources you used, copying text from a source without citation, having someone else write your paper, or buying a paper and submitting it as your own work – are considered plagiarism. These actions represent a misuse of others' intellectual property and are unethical in an academic setting.