Which term best describes an operation that provides security and protection to units?

The term that best describes an operation providing security and protection to units is "Guard." In the context of military operations, a "Guard" is specifically tasked with providing security for units by establishing a defensive position. This operation involves active measures to protect other forces from enemy actions and can include various activities such as maintaining surveillance, setting up defensive fortifications, and engaging potential threats.

In contrast, "Screen" refers to operations that involve observing and reporting enemy activity while providing information but does not primarily focus on the physical protection of units. "Cover" often relates to operations that protect units by engaging the enemy and can also imply a broader supportive role, while a "Task Force" indicates a temporary grouping of forces for a specific mission or objective but does not inherently imply the protective role defined by a "Guard." Each of these terms plays a distinct role in military operations, but "Guard" specifically emphasizes the function of providing direct security and protection for units.