Answer: a research study published on a university’s website about how being in a zoo affects animals.
The most credible source would be the university research study. The site where people share stories is based on hearsay and anecdotal evidence, which is not credible enough. The advertisement does not include research. Moreover, its intention is that of selling something, and not that of informing. Finally, the blog is biased as the zoo owner is likely to have a particular position and advocate for that in his writing. The university research, however, is likely to contain original research and facts that have been proven right.