A polymorphic lookup field is a special type of lookup relationship field that can reference multiple object types.
Option C: The WhatId field on the standard Event object
Correct Answer.
The WhatId field on Event (and Task) can refer to multiple types of objects, such as Account, Opportunity, Custom Objects, etc.
This makes WhatId a polymorphic lookup field.
The ParentId field on Account is a standard lookup to another Account (parent account).
It is not polymorphic.
Option B: The LeadId and ContactId fields on the standard Campaign Member object Incorrect.
Campaign Member uses either LeadId or ContactId, but these are separate fields, not a single polymorphic field.
Option D: A custom field, Link__c, on the standard Contact object that looks up to an Account or a Campaign
*Incorrect, unless it's a polymorphic field, which is not possible for custom fields.
Custom polymorphic lookup fields cannot be created.
Polymorphic lookups are only available on certain standard objects.
Conclusion:
An example of a polymorphic lookup field is Option C, the WhatId field on the standard Event object.
Reference:
Polymorphic Relationships
Incorrect Options:
Option A: The ParentId field on the standard Account object
Incorrect.