Which of the following is a valid instruction when aircraft are taxiing?

When aircraft are taxiing, it is crucial to maintain safety and ensure proper traffic flow on the airfield. Holding at a designated line is a valid instruction because it is essential for aircraft to stop at specific points for various reasons, such as waiting for clearance to enter a runway, yield to other aircraft, or ensure compliance with air traffic control instructions. This instruction helps manage and control the movement of aircraft on the ground, reducing the risk of accidents and ensuring that the sequence of operations is carried out smoothly.

Instructions to proceed across all runways, taxi without restrictions, or assume full clearance are not necessarily valid, as they may not account for the current traffic situation or safety protocols in place at the airport. Each situation must consider air traffic control's guidance to ensure a safe operational environment.