An HTTP 502 Bad Gateway error indicates that the server acting as a gateway or proxy received an invalid response from the upstream server. This suggests there may be an issue with the configuration or availability of the server behind the load balancer. HTTP 502 Error: This error occurs when the load balancer or gateway receives an invalid response from the backend server. Server Configuration: The issue is likely due to misconfiguration or problems with the backend server that needs to be resolved. Option A is correct as it identifies that the issue lies with the server configuration behind the load balancer. Reference: NGINX Documentation: Troubleshooting 502 Errors