Sometimes RTUs refuse to communicate even when the cable is not very long and with good quality. Usually, the RTUs that will suffer are those at the end of the line. This kind of problem can be tackled by one of the following tricks:
In option B the values of the resistor and the capacitor should bein the following range:
Capacitors:1.0 µF; 0.5µF; 0.1µF of 100 volt and with no polarity.
Resistors:22 Ω; 47 Ω; 100 Ω; 200 Ω of 2Watt
By my experience option B is more promising.