You have the Web Server (IIS) role installed on a server that runs Windows Server 2008. You make
changes to the configuration of an application named APP1. Users report that the application fails.
You examine the event log and discover the following error message:
You need to ensure that users are able to connect to APP1.
Which command should you run at the command prompt on the server?

A.
appcmd set config

B.
appcmd stop apppool

C.
appcmd start apppool

D.
appcmd set apppool

Explanation:
To ensure that users are able to connect to App1, you need to run appcmd start apppool on the
server. The “503 Service Unavailable” error mostly occurs whenever HTTP.SYS, the kernel HTTP
driver that manages http connections for IIS, fails to create an IIS worker process to process the
request. This failure is typically caused by a critical error during worker process initialization, or more
likely an unhandled exception / access violation occurring during worker process startup. After a
certain number of failures, the application pool will trigger Rapid Fail Protection, a WAS feature
designed to stop application pools with a persistent failure condition to avoid an endless loop of
failing to start worker processes. At this point, all requests to applications within the stopped
application pool will result in the 503 error, and the application pool will need to be re-started
manually
Reference: Troubleshooting IIS7 503 “Service unavailable” errors with startup debugging
http://mvolo.com/blogs/serverside/archive/2007/05/19/Troubleshooting-IIS7-503-_2200_Serviceunavailable_2200_-errors-with-startup-debugging.aspx