Your load balancer returns HTTP 503. What does this indicate?

1. A The service is currently unavailable, typically because the upstream servers are overloaded, undergoing maintenance, or still in the process of booting up.
2. B The requesting client has provided invalid or missing authentication credentials, preventing the application server from processing the protected resource.
3. C The requested resource could not be found on the server, often signifying a broken link or an incorrect URL path in the client's request payload.
4. D The incoming request body is syntactically malformed or lacks mandatory header fields, causing the server's validation layer to reject the connection.