Which method is used in a test class to indicate the use of mock callouts?

The method used in a test class to indicate the use of mock callouts is Test.setMock. This method allows developers to define a mock response for HTTP callouts made during the execution of tests. When testing code that makes callouts to external services, using mocks is essential to avoid actual network calls, which can lead to unpredictable results and slow down test execution.

By using Test.setMock, you can specify what should be returned when a particular type of callout is made, thereby allowing for controlled and reliable tests. This is especially useful for ensuring that your integration points work correctly under various scenarios without relying on the availability or behavior of external systems.

The other choices do not serve the purpose of establishing mock responses for callouts: Test.startMock and Test.endMock are not valid methods in Salesforce's testing framework, and Mock.setTest does not exist as a method in this context. Thus, the usage of Test.setMock is the appropriate method when preparing for testing with mocked services.