Absolute zero is defined as the temperature at which:

Absolute zero is defined as the temperature at which all kinetic motion ceases. At this theoretical temperature of 0 Kelvin (or -273.15 degrees Celsius), the particles in a substance are at their lowest possible energy state. According to the kinetic molecular theory, temperature is a measure of the average kinetic energy of the particles in a substance; therefore, at absolute zero, this energy reaches a minimum, which means that the particles are no longer vibrating or moving in any significant way.

This concept emphasizes that all matter, regardless of its state (solid, liquid, or gas), would exhibit no thermal motion at this extreme temperature. It represents a fundamental limit where the temperature cannot be lowered further according to the laws of thermodynamics. The implications of this temperature extend into various fields of physics, including quantum mechanics, because even at absolute zero, there are quantum mechanical effects at play, but classical motion is effectively halted.

The other options do not accurately describe absolute zero. For instance, while phase transitions in substances can occur at various temperatures, they are not specifically associated with absolute zero. Similarly, thermal processes cannot be maximized at absolute zero since that implies energetic activity, which contradicts the idea that all motion ceases. Lastly, entropy, a measure of