ANS:B - specific gravity of a liquid.

A pycnometer is used for the measurement of specific gravity of a liquid. A pycnometer is a small glass container with a well-fitting stopper, designed for accurately measuring the density or specific gravity of liquids. It works by weighing a known volume of liquid and comparing it to the weight of an equal volume of water at a standard temperature. This allows for the determination of the liquid's density or specific gravity relative to water. Therefore, the correct choice is: specific gravity of a liquid