Which response code indicates that a request was unauthorized?

The response code that indicates a request was unauthorized is 401 - Unauthorized. This status code is part of the HTTP specification and is specifically used to denote that the request has not been applied because it lacks valid authentication credentials for the target resource.

When a server returns a 401 status code, it prompts the client to provide the necessary authentication details, such as a username and password, to proceed. This code is commonly encountered in scenarios where access control is enforced, ensuring that only authenticated users can access specific resources or services.

In contrast, the other response codes serve different purposes. For instance, a 400 - Bad Request indicates that the server cannot process the request due to a client error (e.g., malformed request syntax). A 429 - Too Many Requests code signifies that the user has sent too many requests in a given timeframe, resulting in a temporary restriction. Meanwhile, a 404 - Not Found indicates that the server cannot find the requested resource. Hence, the unique role of the 401 status code clearly points to the absence of proper authorization for access.