What are the factors an employer can legally consider when determining if a requested accommodation is reasonable?

please cite 5 scholarly references three peer-reviewed journal articles for research to support contentions (and given credit with in-text citations and in the reference listing (in APA format). Using specific examples, no plagiarism, no grammar errors, very detailed information, straight to the point.
1.) What are the factors an employer can legally consider when determining if a requested accommodation is reasonable?