If the ammeter shows complete deflection to the right and the voltage light is on, what is the first action to take?

When the ammeter shows complete deflection to the right and the voltage light is illuminated, this indicates a potential over-voltage condition in the electrical system. The immediate and prudent action in such a situation is to turn the master switch OFF. This action effectively disconnects all electrical components and protects the aircraft's electrical system from potential damage due to excess voltage.

Turning off the master switch ensures that all electrical loads are removed from the system, which helps to prevent electrical failures or fires that could arise from the over-voltage condition. Additionally, shutting down the electrical system provides a safety measure while assessing the cause of the abnormal readings.

In the context of the other choices, while some actions, such as turning off nonessential electrical equipment, may help reduce the load, they do not address the root issue of the over-voltage situation comprehensively. Moreover, terminating the flight or attempting to turn off the over-voltage light would not be appropriate responses directly related to managing the immediate risk posed by the showing of complete deflection and the illuminated voltage light.