Which file contains a list of services and hosts that will be denied by a TCP Wrapper such as tcpd?

Answer: /etc/hosts.deny

Explanation:
From the man pages:
The access control software consults two files. The search stops at the first match:
Access will be granted when a (daemon,client) pair matches an entry in the /etc/hosts.allow file.
Otherwise, access will be denied when a (daemon,client) pair matches an entry in the /etc/hosts.deny file.
Otherwise, access will be granted.
A non-existing access control file is treated as if it were an empty file. Thus, access control can be turned off by providing no access control files.