TCP Wrappers can control when a UDP server starts but has little control afterwards because UDP packets can be sent randomly.
The following answers are incorrect:
It stops packets before they reach the application layer, thus confusing some proxy servers. Is incorrect because the TCP Wrapper acts as an ACL restricting packets so would not confuse a proxy server because the packets would not arrive and would not be a limitation.
The hosts. access control system requires a complicated directory tree. Is incorrect because a simple directory tree is involved.
They are too expensive. Is incorrect because TCP Wrapper is considered open source with a BSD licensing scheme.