The correct answer is C. Set the created date of a record using a system method.

Here's a breakdown of each scenario:

A. Generate a Visualforce PDF with getContentAsPDF(): This is invalid in unit tests. Methods like getContent() or getContentAsPDF() that require making real HTTP requests or interacting with the UI are not allowed in tests because unit tests should run without reliance on external factors like the rendering of Visualforce pages.

B. Load data from a remote site with a callout: This is also invalid in unit tests. Salesforce requires the use of mock callouts in test methods to simulate callouts, rather than performing real HTTP callouts.

C. Set the created date of a record using a system method: This is valid in unit tests. You can use Test.setCreatedDate(recordId, datetime) to manipulate the created date for testing purposes in Salesforce unit tests.

D. Execute anonymous Apex as a different user: This is invalid for unit tests. Unit tests run in the context of a test execution and do not allow for executing anonymous code or switching users arbitrarily....

C.
https://developer.salesforce.com/docs/atlas.en-us.apexref.meta/apexref/apex_methods_system_test.htm#apex_System_Test_setCreatedDate