Answer:
D. Running 50 m along a straight track
Step-by-step explanation:
Running 50 m along a straight track is a real-world example of linear motion.
Linear motion means moving along a (straight) line, a direct and straight line from point A to point B.
All the other proposed answers (A,B,C) involve a curve. The two rock examples will curve due to gravity and running around a curve on a track isn't a straight line.
I hope that helps you understanding the principle of linear motion.