Introduction:
An ideal refrigerant is a substance or fluid that is used in a refrigeration cycle to transfer heat from a low-temperature area to a high-temperature area. It undergoes phase changes from a liquid to a vapor and vice versa during the refrigeration process. An ideal refrigerant should possess certain properties to ensure efficient and effective refrigeration.

Explanation:
The given options describe various characteristics of an ideal refrigerant. Let's analyze each option to determine which one is not true.

a) High latent heat of vaporization and specific heat:
- Latent heat of vaporization is the amount of heat energy required to convert a substance from a liquid to a vapor at a constant temperature.
- Specific heat is the amount of heat energy required to raise the temperature of a substance by a certain amount.
- An ideal refrigerant should have a high latent heat of vaporization and specific heat to absorb and release a significant amount of heat during the phase change process. This helps in efficient heat transfer and cooling.
- This option is true and does not contradict the characteristics of an ideal refrigerant.

b) Critical pressure and temperature well above the maximum operating pressure and temperature limits:
- The critical pressure and temperature of a substance are the values at which it exists as a supercritical fluid, with properties of both a liquid and a gas.
- An ideal refrigerant should have critical pressure and temperature values that are well above the maximum operating limits to ensure safe and reliable operation of the refrigeration system.
- This option is also true and aligns with the requirements of an ideal refrigerant.

c) Low value of specific volume:
- Specific volume is the volume occupied by a unit mass of a substance.
- An ideal refrigerant should have a low specific volume to minimize the size and cost of the refrigeration system. A lower specific volume allows for more efficient compression and expansion of the refrigerant.
- This option is true and corresponds to the desired property of an ideal refrigerant.

d) High value of thermal conductivity:
- Thermal conductivity is the ability of a substance to conduct heat.
- An ideal refrigerant should have a high thermal conductivity to facilitate efficient heat transfer between the refrigerant and the surrounding environment. This helps in rapid cooling and improved refrigeration performance.
- This option is also true and reflects the desired property of an ideal refrigerant.

Conclusion:
Among the given options, option 'A' is not true in respect of an ideal refrigerant. An ideal refrigerant should have a high latent heat of vaporization and specific heat, critical pressure and temperature well above the maximum operating limits, low specific volume, and high thermal conductivity.