Final answer:
In the given options, all three would constitute plagiarism if the source of the information is not identified and cited.
Using someone else's ideas or work without proper attribution, including the use of an author's opinion to support your own viewpoint without citation, constitutes plagiarism. Properly crediting sources is essential to avoid plagiarism and respect intellectual property rights.
Step-by-step explanation:
In the context of academic writing, plagiarism is defined as the act of using someone else's work or ideas without proper attribution. This can occur in various forms, including copying and pasting text without quotes or citation, submitting a friend's paper as your own, or downloading a paper online and claiming it as your original work. Some of the examples posed in the question would indeed be considered plagiarism if the source of the information is not identified and cited. For instance, using the opinion of an author you agree with to summarize your own point of view requires proper citation to differentiate the original author's ideas from your own. Similarly, even if unintentional, a researcher is liable for plagiarism when they omit the necessary credit to their sources, be they direct quotations, paraphrases, or summaries. Therefore, correctly citing existing sources is crucial in academic work to maintain the integrity of the research and to honor the intellectual property rights of the original authors.