When must electrodes be moistened in electrotherapy procedures?

Electrodes must be moistened specifically in iontophoresis because this procedure involves the delivery of electrically charged substances into the skin through the use of a direct current. Moistening the electrodes enhances conductivity, ensuring a more effective and comfortable treatment by allowing the current to flow more easily through the skin. Proper conductivity is essential for the successful penetration of the therapeutic agents used in iontophoresis.

In contrast, while moisture may sometimes be beneficial in other electrotherapy applications, it is not a requirement for every type of treatment. For instance, short wave diathermy typically does not require moistened electrodes as it utilizes a different method of energy transmission. Therefore, option C is accurate as it identifies the specific context in which moistened electrodes are vital for the efficacy of the treatment being performed.