To avoid plagiarism, the best option from the ones you provided is:
paraphrasing and appropriately identifying your source.
Paraphrasing involves rephrasing the original text in your own words, while properly citing the source ensures that you give credit to the original author. The other options, such as copying content exactly or cutting and pasting from sites, can lead to plagiarism if proper attribution is not given. Making a mental note of sources is helpful, but it is not sufficient on its own to avoid plagiarism. Proper documentation and citation are key.