Spontaneity in the context of chemical thermodynamics means having the potential to proceed without the assistance of an external agency.

Spontaneity is a concept in chemical thermodynamics that refers to the tendency of a reaction or a process to occur on its own without any external intervention. It is important to note that spontaneity does not imply how fast a reaction occurs or whether it occurs instantaneously. Instead, it focuses on whether a reaction or process has the potential to proceed without the need for external assistance.

Explanation:
To understand why the correct answer is option 'D', let's break down the meaning of spontaneity in chemical thermodynamics:

1. Spontaneous vs. Non-spontaneous Reactions:
In chemical thermodynamics, reactions can be classified as spontaneous or non-spontaneous based on the thermodynamic driving force. A spontaneous reaction is one that occurs naturally, progressing towards equilibrium without any external influence. On the other hand, a non-spontaneous reaction requires an external agency or intervention to occur.

2. Gibbs Free Energy (ΔG):
The spontaneity of a reaction is determined by the change in Gibbs free energy (ΔG) of the system. The Gibbs free energy is a thermodynamic potential that combines both enthalpy (ΔH) and entropy (ΔS) contributions.

3. ΔG and Spontaneity:
The sign of ΔG determines the spontaneity of a reaction:
- If ΔG < 0:="" the="" reaction="" is="" spontaneous="" and="" will="" proceed="" without="" the="" need="" for="" external="" assistance.="" the="" system="" undergoes="" a="" decrease="" in="" free="" energy,="" indicating="" that="" the="" reaction="" is="" energetically="" />
- If ΔG > 0: The reaction is non-spontaneous and will not occur without an external agency or intervention. The system would need an input of energy to proceed.
- If ΔG = 0: The system is at equilibrium, and the reaction neither proceeds forward nor backward.

4. Potential to Proceed without External Agency:
Based on the above understanding, option 'D' is the correct answer. Spontaneity, in the context of chemical thermodynamics, means that a reaction or process has the potential to proceed without the need for any external agency or intervention. It does not refer to the speed or instantaneousness of the reaction but rather focuses on the inherent tendency of the system to undergo a change in free energy and move towards equilibrium.

In conclusion, spontaneity in chemical thermodynamics signifies the potential of a reaction or process to occur without the assistance of an external agency. It is determined by the change in Gibbs free energy (ΔG) of the system, with a negative ΔG indicating spontaneity and a positive ΔG indicating non-spontaneity.