What method of control should an observer use to inform the FDC of the total time to engage a target?

The method of control that an observer should use to inform the Fire Direction Center (FDC) of the total time to engage a target is "Duration." This term specifically refers to the total length of time designated for an engagement, which encompasses the entire process from the initiation of the fire mission to the completion of the last round impacting the target.

When discussing fire support and combat operations, understanding the duration allows for proper timing in coordination with other assets and maneuvers on the battlefield. By specifying the duration, the observer clearly communicates how long the fire mission should take, enabling the FDC to calculate the necessary firing solutions and manage resources effectively.

In contrast, the other options refer to related concepts but do not adequately capture the total time needed for the engagement. "Time on Target" refers to the specific moment when munitions are expected to arrive at the target, while "Time" and "Interval" describe different aspects of timing that do not convey the comprehensive period required for the entire action of engaging the target. Thus, "Duration" is the most precise and accurate term for communicating the total time needed for the engagement.