Which HTTP requests do not change the server state when repeated?

HTTP requests that do not modify the server's state when repeated are called idempotent and safe. These include:

• GET — used to retrieve data, does not change the server's state.
• HEAD — similar to GET, but only returns headers.
• OPTIONS — requests supported methods for a resource, does not change the state.
• TRACE — returns diagnostic information, does not change the state.

Of these methods, GET is most commonly used for safe data retrieval without changes on the server.