According to the International System of Units (SI), which of the following is the SI base unit of temperature used in science and engineering?

Introduction / Context:
This question checks your knowledge of the SI system of units, which is the standard in science and engineering. Temperature can be measured in several scales in everyday life, such as Celsius and Fahrenheit, but only one of them is the official SI base unit. Correctly identifying this unit is important for understanding thermodynamics, heat transfer and many scientific formulae where absolute temperature appears, such as the ideal gas law PV = nRT.

Given Data / Assumptions:

• The options include Kelvin, degree Celsius, degree Fahrenheit and joule.
• We are asked about the SI base unit of temperature.
• We assume current SI definitions where base units are clearly listed.
• Joule is known as the SI unit of energy, not temperature.

Concept / Approach:
The SI system defines seven base units, including the kelvin for thermodynamic temperature. Kelvin measures absolute temperature starting from absolute zero, the theoretical point where all classical thermal motion ceases. While degree Celsius is widely used for everyday weather and laboratory work, it is derived from the Kelvin scale by a simple offset: T in Celsius equals T in Kelvin minus 273.15. Degree Fahrenheit is used in some countries for weather reporting but is not part of SI. Joule is the SI unit of energy, not temperature. Therefore, the correct SI base unit of temperature is kelvin.

Step-by-Step Solution:
Step 1: Recall the list of SI base units, which includes the metre, kilogram, second, ampere, kelvin, mole and candela. Step 2: Identify that kelvin is specifically designated as the base unit for thermodynamic temperature. Step 3: Recognise that degree Celsius is a commonly used scale, but it is derived from the Kelvin scale and is not itself the SI base unit. Step 4: Note that degree Fahrenheit is used mainly in some countries for weather and cooking and is not part of the SI system. Step 5: Understand that joule is the SI unit of energy and appears in equations for work and heat, not as a unit of temperature. Step 6: Conclude that the correct answer is kelvin.

Verification / Alternative check:
If you look at fundamental equations such as the ideal gas law PV = nRT, the temperature T must be in Kelvin for R (the gas constant) to have its standard SI value. Similarly, in thermodynamic formulas involving entropy and free energy, temperature is always considered in Kelvin. Reference charts of SI base units also explicitly list the symbol K as the unit of thermodynamic temperature. These consistent uses confirm that kelvin is the official SI unit of temperature.

Why Other Options Are Wrong:
Degree Celsius is not an SI base unit; it is derived from the Kelvin scale by a fixed offset, even though it is widely used in daily life. Degree Fahrenheit is a non SI scale associated with older measurement systems and is not used in scientific SI based work. Joule is the SI unit of energy, not temperature, and appears in contexts involving work and heat rather than in temperature measurement directly.

Common Pitfalls:
Students may mistakenly choose degree Celsius because they are more familiar with it and see it used in many laboratory thermometers. To avoid this, remember that the SI system focuses on absolute physical quantities, and Kelvin is the only base temperature unit. Also recall that when performing calculations involving thermodynamic formulas, you almost always convert temperatures to Kelvin, not to Celsius or Fahrenheit.

Final Answer:
The SI base unit of temperature is the Kelvin (symbol K).