Which of the following are credible sources of information for a research paper?
an article published in an entertainment magazine
a study by researchers published on a university website
a book that gives information in simple language
a book written by a scholar on the topic
an article on a company's website

I'd say there are two options here which could be credible sources of information for a research paper, and those are a study by researchers published on a university website and a book written by a scholar on the topic. If researchers have been studying this topic for a long time, and if the university thinks it is good enough to post it on their website, it is definitely reliable. Also, scholars research topics thoroughly, so their studies can also be trusted. The other options are not as credible.