In the International System of Units (SI), the kelvin (K) is used as the standard unit for measuring which physical quantity?

Introduction / Context:
The International System of Units, or SI system, defines standard units for all basic physical quantities so that scientists and engineers around the world can communicate measurements consistently. The kelvin, symbol K, is one of the seven base SI units. The question checks whether you know which physical quantity is measured in kelvin. This is a fundamental fact that appears in physics, chemistry, and engineering, especially in topics related to thermodynamics, gases, and heat transfer.

Given Data / Assumptions:

• The unit under discussion is kelvin, symbol K.
• Kelvin is known to be an SI base unit.
• Possible physical quantities listed are density, pressure, mass, and temperature.
• We assume standard SI usage as taught in school level physics and chemistry.

Concept / Approach:
Kelvin is the SI base unit of thermodynamic temperature. It is defined based on the triple point of water and, in modern definitions, related to fundamental physical constants. Temperature scales such as Celsius and Fahrenheit are more common in everyday life, but scientific work prefers kelvin because it starts from absolute zero, the theoretically lowest possible temperature. Density has SI units of kilogram per cubic meter. Pressure has SI units of pascal, which is newton per square meter. Mass has SI units of kilogram. Therefore, among the choices, temperature is the only physical quantity correctly associated with kelvin as its SI base unit.

Step-by-Step Solution:
Step 1: Recall the list of SI base units: metre, kilogram, second, ampere, kelvin, mole, and candela.Step 2: Associate each base unit with its corresponding physical quantity; kelvin is paired with thermodynamic temperature.Step 3: Identify that density is a derived quantity with units kg per m^3.Step 4: Recognise that pressure is also derived, with unit pascal, equal to newton per m^2.Step 5: Conclude that temperature is the correct physical quantity measured in kelvin.

Verification / Alternative check:
Check standard scientific formulas. The ideal gas law is written as pV = nRT, where temperature T must be expressed in kelvin for the equation to work correctly in SI units. Also, many thermodynamic relations use absolute temperature in kelvin because absolute zero is a natural reference point. Everyday Celsius readings are converted to kelvin by adding 273 approximately, which again confirms that kelvin is a temperature unit. No such association exists between kelvin and density, mass, or pressure, which makes temperature the only consistent choice.

Why Other Options Are Wrong:
Density is typically measured in kilograms per cubic meter or grams per cubic centimeter, not in kelvin, so option A is wrong. Pressure is measured in pascals, bars, or atmospheres in various contexts, but not in kelvin, so option B is incorrect. Mass is measured in kilograms, grams, or related units rather than kelvin, so option C is also wrong. Only option D, temperature, correctly matches the SI base unit kelvin.

Common Pitfalls:
Some students confuse kelvin with the Celsius scale because both appear in temperature related problems. However, Celsius is not an SI base unit, while kelvin is. Another pitfall is confusing base units and derived units. Remember that temperature is so fundamental that it has its own base unit, while quantities like density and pressure are combinations of mass, length, and time units. Keeping a mental list of SI base units and their quantities helps avoid errors in multiple choice questions about units.

Final Answer:
Temperature