Which of the following opioids cause the least amount of nausea?

Intraoperative use of opioids is a well-known risk factor for postoperative nausea and vomiting. Opioids stimulate the chemoreceptor trigger zone in the area postrema of the medulla, possibly through d-receptors, and thereby lead to nausea and vomiting. When compared with approximately equipotent doses of fentanyl and sufentanil, alfentanil is associated with a lower incidence of postoperative nausea and vomiting Ref: Miller's anesthesia-7th ed, Chapter 27.