The intensity of an earthquake is measured by:

Scientific instruments are designed and named for the specific physical quantity or signal they are built to detect. Ground shaking caused by the sudden release of energy along a fault travels outward as seismic waves; the instrument that detects and continuously records this ground vibration is called a seismograph (or seismometer), and its record, the seismogram, is what seismologists use to work out how strong an earthquake was. Strictly speaking, seismologists distinguish two related quantities: magnitude, an instrument-derived measure of the energy released, and intensity, a location-dependent measure of the shaking's observed effects (rated on a scale such as the Modified Mercalli scale). Even so, the seismograph is the instrument through which earthquake ground-motion is actually detected and measured, so in standard general-knowledge and competitive-exam usage it is treated as the instrument that measures the intensity of an earthquake.

Among the four listed instruments, only the seismograph is built to sense the ground vibration produced by an earthquake, so, by this standard convention, it is the instrument used here to measure the intensity of an earthquake.

• Polygraph: records physiological signals (pulse, blood pressure, breathing, skin conductivity) of a person; it has no sensor for ground motion and is used in lie-detection, not seismology.

• Barometer: measures atmospheric pressure and is used for weather forecasting and altitude estimation, not for detecting ground shaking.

• Hydrometer: measures the density (specific gravity) of a liquid by flotation; it has nothing to do with vibration or seismic waves.

So, by standard convention, the instrument used to measure the intensity of an earthquake is the seismograph.