Explanation:
The correct answer is option 'B' which states that Beckmann thermometers are used to measure the elevation in boiling point or depression in freezing point.
Here's a detailed explanation of why this is the correct answer:

Beckmann Thermometer:
A Beckmann thermometer is a type of liquid-in-glass thermometer that is specifically designed to measure small temperature differences accurately. It is named after the German chemist Ernst Otto Beckmann who invented it in the late 19th century.

Measurement of Elevation in Boiling Point or Depression in Freezing Point:
The Beckmann thermometer is primarily used to measure the elevation in boiling point or the depression in freezing point of a solution. This is achieved by comparing the boiling or freezing point of the solution with that of a pure solvent.

Elevation in Boiling Point:
When a solute is added to a solvent, the boiling point of the resulting solution increases. This is known as the elevation in boiling point. The degree of elevation depends on the concentration of the solute. The Beckmann thermometer can accurately measure this elevation in boiling point.

Depression in Freezing Point:
Similarly, when a solute is added to a solvent, the freezing point of the resulting solution decreases. This is known as the depression in freezing point. Again, the degree of depression depends on the concentration of the solute. The Beckmann thermometer can accurately measure this depression in freezing point.

Working Principle of a Beckmann Thermometer:
The Beckmann thermometer consists of a long capillary tube filled with a special liquid, usually mercury. The liquid expands or contracts with temperature changes, and the expansion or contraction is measured by a scale attached to the thermometer.

Calibration:
To use a Beckmann thermometer, it needs to be calibrated. This is done by measuring the temperature difference between the thermometer bulb and the end of the mercury column. This calibration is essential to ensure accurate measurements.

Applications:
The Beckmann thermometer finds applications in various fields, including chemistry, biochemistry, and pharmaceuticals. It is commonly used to determine the molecular weight of substances, as well as to measure the concentration of solutes in solutions.

In conclusion, Beckmann thermometers are specifically designed to measure the elevation in boiling point or depression in freezing point of solutions. This makes them a valuable tool in various scientific disciplines.