Explanation:
To understand why the correct answer is option 'D', let's first define the terms involved in the question.

Entropy:
Entropy is a measure of the disorder or randomness of a system. It is denoted by the symbol S. An increase in entropy represents an increase in disorder, while a decrease in entropy represents a decrease in disorder.

Enthalpy:
Enthalpy is the heat content of a system at constant pressure. It is denoted by the symbol H. An exothermic reaction releases heat and has a negative enthalpy change, while an endothermic reaction absorbs heat and has a positive enthalpy change.

Internal Energy:
Internal energy is the sum of the kinetic and potential energies of the particles in a system. It is denoted by the symbol U. The internal energy of a system can change due to heat transfer or work done on or by the system.

Free Energy:
Free energy is a thermodynamic potential that measures the maximum reversible work that can be performed by a system at constant temperature and pressure. It is denoted by the symbol G. A spontaneous reaction is one that occurs on its own without any external influence.

Spontaneity of a Reaction:
For a reaction to be spontaneous, it must proceed in the forward direction without any external influence. The spontaneity of a reaction is determined by the change in free energy (∆G) of the system.

∆G and Spontaneity:
The change in free energy (∆G) is related to the change in enthalpy (∆H) and the change in entropy (∆S) of the system by the equation: ∆G = ∆H - T∆S, where T is the temperature in Kelvin.

Explanation of the Correct Answer:
According to the equation ∆G = ∆H - T∆S, for a reaction to be spontaneous (∆G < 0),="" either="" ∆h="" must="" be="" negative="" (exothermic="" reaction)="" or="" ∆s="" must="" be="" positive="" (increase="" in="" disorder).="" />

If the reaction is accompanied by a decrease in entropy (∆S < 0),="" option="" 'a'="" is="" incorrect="" because="" a="" decrease="" in="" entropy="" would="" make="" the="" reaction="" less="" />

If the reaction is accompanied by a decrease in enthalpy (∆H < 0),="" option="" 'b'="" is="" incorrect="" because="" a="" decrease="" in="" enthalpy="" alone="" does="" not="" guarantee="" spontaneity.="" the="" change="" in="" entropy="" also="" plays="" a="" crucial="" />

If the reaction is accompanied by a decrease in internal energy (∆U < 0),="" option="" 'c'="" is="" incorrect="" because="" the="" change="" in="" internal="" energy="" does="" not="" directly="" determine="" the="" spontaneity="" of="" a="" reaction.="" it="" is="" the="" change="" in="" free="" energy="" that="" determines="" />

Therefore, the correct answer is option 'D' - a chemical reaction will be spontaneous if it is accompanied by a decrease in free energy of the system. This means that the reaction releases energy and tends to proceed in the forward direction on its own without any external influence.