Select the option that best completes the analogy: Barometer : Pressure :: Seismograph : ?

The relation is instrument : the thing that instrument records. A barometer records atmospheric pressure; a seismograph records earth tremors. The distractors are quantities recorded by other instruments — rainfall by a rain gauge, humidity by a hygrometer, wind speed by an anemometer — so all three feel plausible until the instrument named is matched with its own reading.