To solve this question, the user needs to have knowledge of webMethods Integration Server and how it handles communications.
Option A is incorrect. While ports 5555, 8021, 80, and 443 can be used for listeners, they are not the only ports available for use.
Option B is incorrect. Port numbers greater than or equal to 1000 are not the only ports available for use.
Option C is correct. The webMethods Integration Server can have an active listener on any valid port number that is not already in use.
Option D is incorrect. While ports 21, 80, and 443 can be used for listeners, any port number above 1000 is not guaranteed to be available for use.
Therefore, the answer is: C. Any valid port number not already in use.