Answer: Money
Explanation:
None of the others fit without more reasoning. If she sees more then $20 is there she can change the data using money.
Which device would help someone train for a marathon?
Drone
Navigation system
Smart watch
VR headset
I think Smart watches will be good as it will track your breathing, heartbeat, and steps. VR head set will not make you go so far, drones can look at the track you are running but it is not helping you run, and i don’t know about navigation systems.