Because we are trying to find the distance traveled, we would usually need velocity, time, acceleration and position, which is where the answer choices come from. Unfortunately, we can only pick one of these choices to use, which makes it a tad bit more difficult.
Let's see what we have from the question. It states that the train is traveling an average speed of 80 km/h, which gives us enough information to eliminate two of the answers.
80 km/h is the speed of the train, or the velocity, so we can cross out A.
Because it says the train travels at an average of 80 km/h, we know that it's not speeding up or slowing down, meaning that it has no acceleration, letting us cross out C.
This leaves us with position and time traveled as our answer choices. There's a special trick you can pull in physics which allows you to mark things according to its relative position, speed or acceleration. In this problem, it doesn't really matter where the train starts, as we're not trying to find out specifically where it ends, we're just trying to find out how far it traveled. This means that we can call wherever this train starts "0 m" and then use its velocity and time to see how far it traveled from that starting mark.
This means the answer should be D) time traveled!
