When writing tests for Apex code that performs callouts to external services, Salesforce requires that callouts are mocked to ensure that tests do not depend on external systems and to allow for consistent and predictable test results.
Correct Option:
Option D: Write a class that implements HttpCalloutMock.
True.
To test HTTP callouts, you implement the HttpCalloutMock interface and provide a fake response for the HTTP request.
This mock response is then used in your test methods to simulate the callout.
The test method uses Test.setMock to set the mock class.
WebServiceMock is used for testing SOAP callouts, not REST (HTTP) callouts.
Since the code uses REST API, HttpCalloutMock is appropriate.
HttpCalloutMock is an interface, not a class. You cannot extend an interface; you implement it.
The correct approach is to implement the HttpCalloutMock interface.
Reference:
Testing HTTP Callouts by Implementing HttpCalloutMock
Incorrect Options:
Option A & C: Write a class that implements or extends WebServiceMock.
False.
Testing SOAP Callouts
Option B: Write a class that extends HttpCalloutMock.
False.
Interface vs. Class
Conclusion:
To properly test REST API callouts in Apex, you should write a class that implements HttpCalloutMock and use it in your test methods.

https://developer.salesforce.com/docs/atlas.en-us.pages.meta/pages/pages_output_pdf_renderas.htm You can generate a downloadable, printable PDF file of a Visualforce page using the PDF rendering service. Convert a page to PDF by changing the <apex:page> tag.
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