In the calculation of a constant temperature process the:

In a constant temperature process, commonly referred to as isothermal processes in thermodynamics, it is essential for the pressure and temperature to be measured in absolute terms. Absolute temperature (measured in Kelvin) is critical because thermodynamic calculations, such as those involving ideal gases, require temperature to be represented on an absolute scale. This ensures that the laws of thermodynamics apply correctly, notably when using equations that govern gas behavior, such as the ideal gas law.

Using absolute pressure is similarly important. When performing calculations involving gas laws, the pressures must be absolute to accurately determine the behavior of the gas. This is vital for ensuring that calculations reflect the true physical conditions within the system.

While volume typically can be measured in relative terms, for thermodynamic processes and when applying the ideal gas calculations, pressures and temperatures must always be in absolute units in order to maintain accuracy in the laws governing the system. This is why the option addressing the need for absolute pressure and temperature is the correct choice for understanding a constant temperature process.