The requirement is to test a trigger without changing the organization's data. Apex test classes are designed for this exact purpose. They run in a separate transaction that is rolled back at the end of the test run, meaning any data created, modified, or deleted during the test does not persist in the database. The Developer Console's Test menu is a standard tool for executing these test classes and verifying trigger logic safely.

A. Deploying a class does not execute or test any code; it only saves the class definition to the organization.

B. Creating a record via the UI will permanently save the new Contact to the database, violating the requirement not to change org data.

D. Execute Anonymous runs code that does commit DML operations to the database, permanently changing the organization's data.

1. Apex Developer Guide, Introduction to Apex Code Test Methods. "By default, Apex tests don’t commit any data to the database... This way, you aren’t left with test data in your organization after the test executes."

2. Apex Developer Guide, Run Apex Test Methods. This section describes using the Developer Console to run tests, stating, "You can run a single test method, the methods in one class, or all test methods in your org."