Profiles in Salesforce serve as the foundation for what a user can "do" with records, specifically defining Object-Level Security (Create, Read, Edit, Delete permissions). The standard Marketing User profile typically does not include full "Edit" access to the Opportunity object by default, as marketing roles are traditionally focused on Leads and Campaigns. To fulfill the requirement of allowing marketers to maintain campaign relationships on Opportunities, the administrator must ensure the profile has the necessary object permissions.
If the organization is using a custom profile based on the Marketing User template, the admin should edit the object permissions directly on that profile to include "Read" and "Edit" for Opportunities. This allows the marketers to view the records and update the "Primary Campaign Source" field. While a Permission Set could also grant this access, the question implies the admin is currently configuring the profile itself. Adjusting the profile's object permissions is the direct way to align the user's capabilities with the business's functional requirements.