Final answer:
A proxy server is the correct answer as it serves as an intermediary between a network host and the Internet, improving security and managing traffic.
Step-by-step explanation:
The computer system or application/service that acts as an intermediary between a network host and the Internet is known as a proxy server. A proxy server helps to improve security and manage the network traffic by routing requests from clients through the server to the internet, and then passing back the response from the internet to the client.
This can provide anonymity for the user and can be used for content filtering, caching data, or bypassing geo-restrictions. The other options such as router, switch, and firewall, serve different functions within a network. Routers route data packets across networks, switches connect devices within a single network and manage data flow within it, and firewalls protect networks by controlling incoming and outgoing network traffic.