A seismometer is an instrument used to detect and measure earthquakes.
By the same relationship, a thermometer is an instrument used to measure temperature.
Mercury is only the substance used inside a thermometer, not the quantity it measures.
Fever is a condition a thermometer can help detect, but it is not the general quantity measured.
A doctor is a person who uses a thermometer, not the quantity measured.
Therefore the correct analogy is: Seismometer : Earthquakes : : Thermometer : Temperature.
Hence the correct option is (2) Temperature.
Note: the table below appeared in the original solution and belongs to a different question; it is kept here only for completeness.
| - ⇒ + | + ⇒ - |
| ÷ ⇒ × | × ⇒ ÷ |