Which substance is a cathodic reactant?

Oxygen is a substance that acts as a cathodic reactant in electrochemical processes, particularly in corrosion mechanisms. In a cathodic reaction, electrons are gained, and the presence of oxygen is critical because it typically participates in reduction reactions. For example, in the corrosion of metals in an aqueous environment, oxygen can be reduced to hydroxide ions (OH⁻) at the cathode of an electrochemical cell. This process is fundamental in understanding cathodic protection systems, where the aim is to control and mitigate corrosion by manipulating the electrochemical reactions that occur on a metal surface.

The other choices do not function as cathodic reactants. Amps (amperes) represent the flow rate of electric current, resistance is a measure of how strongly a material opposes the flow of electricity, and the pH scale measures the acidity or alkalinity of a solution. None of these options are substances that directly participate in the reduction reactions typically seen in cathodic processes.