Choose the word which is different from the rest.

The answer is D. Pyknometer.

A viscometer is an instrument used to measure the viscosity of a fluid. An anemometer is an instrument used to measure wind speed. A spectroscope is an instrument used to study the spectrum of light emitted by a substance. A pyknometer is an instrument used to measure the density of a liquid or solid.

The pyknometer is the only instrument that does not measure a physical property of a fluid. The other three instruments measure the viscosity, wind speed, and spectrum of light, respectively.