Final answer:
A runway incursion involves unauthorized presence on a runway, with the correct answer being entering the runway without authorization from the Control Tower.
Step-by-step explanation:
A runway incursion is an incident where an unauthorized aircraft, vehicle, or person is on a runway which affects the safety of airport operations. The correct answer to what a runway incursion is: 1) Entering the runway without authorization from the Control Tower. This action disrupts the orderly flow of traffic on the airfield and can lead to dangerous situations, including potential collisions. To illustrate, if a vehicle drives onto an active runway without clearance while an airplane is landing or taking off, this would be considered a runway incursion.