What brevity term acknowledges sighting a specified reference point either visually or via sensor?

The correct choice is "Spot." This term is used in military communication to confirm that a specific reference point has been observed, either visually or through the use of a sensor. The use of "Spot" helps to convey to team members and support elements that they have obtained visual confirmation of the target or reference location required for mission execution.

While other options might seem similar at first glance, they convey different specific meanings. "Visual" might indicate that something has been seen, but it does not necessarily imply that it relates to a reference point that requires acknowledgment. "Contact" typically refers to detecting enemy forces or a specific interaction with threats, rather than confirming a reference point. "Joy" is often used to describe a situation where friendly units have achieved visual contact, but it lacks the specificity of confirming the location related to a pre-determined reference point.

Thus, "Spot" is the most precise term for acknowledging the sighting of a specified reference point, ensuring clarity in communication regarding the location observed, which is crucial for effective operations and coordination in a combat environment.