What indication must manual voltage regulators provide when voltage is outside the accepted range?

Manual voltage regulators are designed to provide clear indications to users when the output voltage strays outside the predefined acceptable range. A visual indicator serves this purpose effectively, as it allows users to quickly ascertain voltage status without needing to refer to more complex systems or tools. Visual indicators can take various forms, such as LED lights or gauges. Their immediate visibility is essential, allowing operators to react swiftly in situations where a voltage deviation could lead to equipment damage or other hazards.

In contrast to the other options, while audible alarms and digital displays could certainly provide alerts, they may not be standard features across all manual voltage regulators. Color-coded warnings can also be helpful, but they are not universally used, and thus, a straightforward visual indicator remains the most practical and widely accepted method for conveying essential information regarding voltage levels. This simplicity and directness make it the correct answer in this context.