Hi, welcome to the forum.
You say "sensors", more than one.
You can not put sensors at a distance of 50 meters. You need a second Arduino board near the sensors.
For the switch in scenario "A", I would use a few mA via a optocoupler. The cable does not matter.
For safety and for noise, it is better if one wire is the GND wire.
Without optocoupler, a electric pulse from electrical equipment nearby, could destroy the Arduino board.
The scenario "B" is not possible. Normal digital signals can not go that far.