Which of the following refers to the absence of input voltage?

The correct answer describes an open circuit, which is characterized by the absence of closed paths for current to flow. In an open circuit, there is a break in the continuity of the circuit, often resulting in no input voltage or current reaching the load. Essentially, if a circuit is open, it cannot deliver power, which aligns with the context of the question regarding input voltage presence.

Understanding why this is the correct choice involves recognizing how electrical circuits function. In a proper functioning circuit, the flow of electricity hinges on connections being intact. When those connections are severed (open), the voltage cannot be delivered to any connected component, rendering it inactive.

In contrast, other options do not denote the absence of input voltage. A short circuit refers to an unintended path that allows current to bypass the main load, potentially leading to high current flow and damage. Overload pertains to a situation where the demand on a circuit exceeds its capacity, resulting in excessive current that can cause overheating, while under-voltage indicates that the voltage levels are below expected but still present, thus not matching the condition of complete absence described in the question.