Final answer:
The use of a role hierarchy, combined with organization-wide defaults set to private, would allow Sarah to view all EMEA region opportunities while preventing Peter, head of US sales, from accessing these records.
Step-by-step explanation:
The appropriate security control to use for Sarah to see all opportunities in the EMEA region without allowing Peter, who is head of US sales, to see them is role hierarchy. In a CRM system like Salesforce, the role hierarchy ensures that users higher in the hierarchy automatically have access to the records owned by users below them in the hierarchy. By setting the organization-wide defaults for opportunities to private, Sarah's role can be placed above the EMEA region sales team in the hierarchy, giving her access to their opportunities, while Peter's role would be separate, without privileges to view opportunities in the EMEA region.