The correct answer is C. a resistance increasing with voltage rise across the arc.
An arc is a plasma that forms when a current flows through a gas. The plasma is made up of charged particles, which are attracted to the electrodes at either end of the arc. This attraction causes the current to flow through the arc, and the resistance of the arc increases with the voltage rise across it.
The resistance of an arc is due to the collisions between the charged particles in the plasma. The higher the voltage, the more collisions there are, and the higher the resistance. This is why an arc behaves as a resistance increasing with voltage rise across the arc.
The other options are incorrect because they do not describe the behavior of an arc. A capacitive reactance is a reactance that opposes changes in voltage. An inductive reactance is a reactance that opposes changes in current. A resistance decreasing with voltage rise across the arc is not a physical phenomenon.