Which of the following is indicated by a 400 Series server return code?

In the context of HTTP status codes, a 400 Series return code indicates that a client error has occurred. These codes represent situations where the server is not responsible for the error that has taken place; instead, the issue stems from the request made by the client. For example, a common status code such as 404 indicates "Not Found," signifying that the server could not find the requested resource due to an error in the client's input.

The 400 status code range serves as a way for servers to communicate that the request was not completed successfully because of a problem with the client's request. It allows clients to understand that they need to correct their request, effectively informing them that the onus is on the client to resolve the issue.