Which module does MS IIS use to connect to the application server?

The correct answer is isapi_redirect.dll, which is a key component for enabling communication between Microsoft Internet Information Services (IIS) and an application server, such as Apache Tomcat. This module functions as a connector that allows IIS to forward requests to the application server, facilitating the handling of dynamic web content. It utilizes the ISAPI filter mechanism to intercept requests, enabling the application server to process them efficiently.

isapi_redirect.dll operates by mapping URLs to specific resources in the application server and managing the transmission of requests and responses. This effectively extends the capabilities of IIS, allowing it to serve as a front-end web server while delegating the processing of application logic to the back-end application server.

Other options, while relevant in different contexts, do not serve the purpose of connecting IIS directly to an application server as effectively as isapi_redirect.dll. For instance, mod_jk is primarily used with Apache HTTP Server to connect it to Tomcat, while appian-connector.dll and socket-connector.dll are specific to different integration or communication protocols, but not for the direct connection between IIS and an application server.