Final answer:
Unified Threat Management (UTM) provides the broadest protection by combining multiple security features like firewall, antivirus, and intrusion detection into a single device, offering a comprehensive and layered approach to security.
Step-by-step explanation:
The appliance that provides the broadest protection by combining several security functions is d) Unified Threat Management (UTM). A UTM is a comprehensive solution that integrates multiple security features into a single device. These features typically include a firewall, antivirus, anti-spyware, anti-spam, intrusion detection and prevention systems, and sometimes even content filtering.
Unlike standalone devices that offer one specific type of protection, UTMs deliver a layered approach to security which can simplify management and increase efficiency. Because of the integration of these multiple functions, UTMs are well-suited for protecting against a wide range of threats, making them an ideal choice for businesses seeking to simplify their security infrastructure.