From the Apache access log entry, which status code indicates that the response was not successful?

The status code 500 signifies an internal server error, indicating that the server encountered an unexpected condition that prevented it from fulfilling the request. This response is a clear indication that something went wrong on the server side, leading to an unsuccessful attempt to complete the requested action.

A 200 status code, on the other hand, represents a successful HTTP request, meaning that the server successfully processed the request, and the desired resource is being returned. Similarly, a 404 status code signifies that the requested resource could not be found on the server, which, although it indicates a failure to locate the resource, does not imply an issue within the server itself. The 403 status code indicates that the server understood the request but refuses to authorize it, which means the request was received but not fulfilled due to permissions, making it a different form of an unsuccessful response as well.

In the context of evaluating HTTP status codes specifically linked to server errors, the 500 code clearly indicates a failure from the server's end, emphasizing why this is the correct choice in identifying a non-successful response.