To check the test coverage of active Process Builder and Flows deploying them in a Change Set, a developer can use SOQL and the Tooling API. The Tooling API provides the FlowTestCoverage object, which represents the test coverage for a flow or process by a given Apex method. The developer can use SOQL queries to retrieve the FlowTestCoverage records for the flows and processes that are included in the Change Set, and calculate the overall test coverage percentage. The developer can also use the Tooling API to create, update, or delete FlowTestCoverage records.
Option A is incorrect because the Flow properties page does not show the test coverage of the flow or process.
Option B is incorrect because the code Coverage Setup page only shows the test coverage of Apex classes and triggers, not flows or processes. Option C is incorrect because the Apex testresult class only provides information about the test execution results, not the test coverage of flows or processes.
References: FlowTestCoverage | Tooling API | Salesforce Developers, How to Invoke Apex from Flows Using InvocableMethod Annotation, How can a developer check the test coverage of active process builder and flows deploying them in a changing set?