What should be done if no visual on an aircraft is observed by the tower?

The correct choice involves the safety protocols for air traffic control when visibility of an aircraft is compromised. If the tower has no visual contact with an aircraft, it is crucial to maintain safe separation between aircraft to prevent potential collisions. This is why canceling separation procedures is necessary; it enables air traffic control to manage the situation without risking the safety of other aircraft in the vicinity.

Maintaining separation procedures allows controllers to provide safe distances and altitudes for aircraft movement, ensuring that operations can continue smoothly while prioritizing safety. It is essential during situations where visual confirmation is lacking. This may involve issuing instructions for holding patterns to keep aircraft at a safe distance from one another until visibility is restored or the situation is clarified.

Other options, such as authorizing immediate takeoff or notifying a pilot to abort takeoff, would not address the lack of visual contact and could lead to dangerous situations by allowing aircraft to take off or operate without proper situational awareness. Issuing holding instructions is the most responsible action in this context, as it directly addresses the safety concerns arising from the loss of visual contact with an aircraft.