The correct option is Pascal (Pa) - SI unit of pressure
Explanation
Pressure is defined as the force exerted per unit area. In scientific measurements, units are categorized into standard International System (SI) units and other common non-SI units used for specific practical applications.
Analysis of Options
- Option (a) is Correct: The Pascal (Pa) is the designated SI unit of pressure. It is defined mathematically as one Newton per square meter ($1 \text{ Pa} = 1 \text{ N m}^{-2}$).
- Option (b) is Incorrect: The Atmosphere (atm) is a traditional unit used to measure pressure, especially in chemistry and meteorology, but it is not the SI standard.
- Option (c) is Incorrect: While the Pascal is the standard unit, it represents a very small amount of pressure. Therefore, the Atmosphere (atm) or bar is generally referred to as the "common unit" for measuring gas pressure in practical contexts.
- Option (d) is Incorrect: The conversion relationship between atmosphere and Pascal is specific. $1 \text{ atm}$ is equal to 101,325 Pa ($1.01325 \times 10^5 \text{ Pa}$), not 100 Pa.
Key Takeaway:
The Pascal (Pa) is the SI unit of pressure. The Atmosphere (atm) is a common non-SI unit, related to the SI unit by the equation $1 \text{ atm} = 101,325 \text{ Pa}$.