Which term refers to a situation where a pilot is unable to see an aircraft in the formation?

The term that refers to a situation where a pilot is unable to see an aircraft in the formation is "No Joy." This terminology indicates that the pilot does not have visual contact with another aircraft, which is crucial in formation flying for maintaining situational awareness and safety.

In the context of formation flying, clear communication about visibility is essential, as pilots must constantly be aware of the positions of their wingmates to avoid collisions and ensure cohesion. When a pilot calls out "No Joy," it alerts other members of the formation to the fact that they should check their own visual references and communicate appropriately to re-establish visual contact if necessary.

Other terms in the choices have specific meanings in aviation, but they do not indicate the inability to see another aircraft. For example, "Visual" means that the pilot can see the aircraft, while "Tally Ho" indicates that the pilot has visual contact with a target. "Padlocked" describes a situation where a pilot is visually fixated on a target, making it difficult to maintain awareness of other aircraft, but it does not imply that the pilot cannot see the formation member at all. Thus, "No Joy" accurately captures the scenario described in the question.