An ohmmeter is used to measure what?

An ohmmeter is specifically designed to measure electrical resistance, which is the opposition that a material offers to the flow of electric current. Resistance is expressed in ohms, hence the name "ohmmeter." This instrument does so by applying a small voltage across the component being tested and measuring the resulting current. The relationship between voltage, current, and resistance is described by Ohm's Law, which is fundamental in understanding electrical circuits.

The other options relate to different measurements: temperature is measured in degrees, frequency refers to the number of occurrences of a repeating event per unit time, and hertz is a unit of measurement for frequency. Each of these parameters is measured using specific instruments designed for those purposes, and thus they do not pertain to the functionality of an ohmmeter.