Final answer:
A Unified Threat Management (UTM) device provides the broadest protection by combining multiple security functions, including firewall services, antivirus, anti-spam, and VPN capabilities.
Step-by-step explanation:
The appliance that provides the broadest protection by combining several security functions is D. Unified Threat Management (UTM) device. Unlike singular solutions like a firewall, which only blocks unauthorized access to a network, or antivirus software, which protects against malware, UTM devices integrate multiple security features. These features can include firewall services, antivirus, anti-spam, a Virtual Private Network (VPN), and more. This consolidation means that a UTM provides comprehensive security measures to protect against a wide variety of threats, making it an efficient and effective choice for managing an organization's security.