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

The method of control that an observer would use to inform the Fire Support Coordinator (FDC) of the total time to engage a target is known as "Duration." This term refers specifically to the total amount of time that the fire support will be active on the target. By communicating the duration, the observer provides the FDC with critical information necessary for coordinating the timing of munitions delivery and ensuring that the effects on target align with broader operational objectives.

This is distinct from the other options. "Time on target" conveys when the munitions should impact, and is important for specific timing, but it does not encompass the total time involved in the engagement. "Time" on its own is too vague, lacking the specificity of engagement continuity. "Interval" refers to the time gap between rounds or phases of fire, which also does not accurately represent total engagement time. Therefore, "Duration" is the most precise method for communicating the total time required for engaging a target in a fire support scenario.