To answer this question, the user needs to be familiar with IIS and its features. The feature for monitoring the health of multiple worker processes servicing an application pool is called Rapid-fail protection.

A. HTTP keep-alive: This is a feature that allows multiple requests to be sent over the same TCP connection, which can improve performance by reducing the overhead of setting up and tearing down connections. However, it is not related to monitoring the health of worker processes.

B. IS monitoring services: This is not a feature in IIS. Therefore, it is incorrect.

C. Application failsafe: This is not a feature in IIS. Therefore, it is incorrect.

D. Rapid-fail protection: This feature monitors the status of worker processes and disables any that are not responding or are experiencing errors. This helps to ensure that users are not routed to a failing worker process, which can improve the reliability and availability of the application. Therefore, the correct answer is D.

The Answer is: D