Which command is described as having a broad continuing mission and includes forces from two or more military departments?

The correct choice is Unified Combatant Command. This command structure is designed to conduct operations and provide strategic direction across multiple military branches, such as the Army, Navy, Air Force, and Marines, ensuring coordinated efforts in diverse and expansive missions. Unified Combatant Commands are established to address specific regional or functional areas, allowing for the effective integration of varying capabilities and resources from the different military departments.

This structure is crucial for missions that require a combined effort of various service branches, thereby facilitating better coordination and efficiency in operations that span across different areas of expertise and capability. The broad continuing mission ensures that these commands are always prepared for a range of operations, from combat to humanitarian assistance, under one unified command.