Final answer:
Combustion reactions require more than just favorable thermodynamics to occur. They also require the presence of a catalyst, sufficient activation energy to initiate the reaction, and adequate oxygen availability.
Step-by-step explanation:
Combustion reactions occur when a substance reacts with oxygen gas, releasing energy in the form of light and heat. The dG (Gibbs free energy change) of glucose and methane combustion reactions is negative, indicating that the reactions are thermodynamically favorable. However, combustion reactions require more than just favorable thermodynamics to occur. They also require the presence of a catalyst, sufficient activation energy to initiate the reaction, and adequate oxygen availability. Therefore, options a) Oxygen availability is limited, b) Combustion requires a catalyst, c) Activation energy is not reached, and d) Thermodynamics alone does not determine reaction kinetics, all play a role in why glucose and methane do not combust readily in our atmosphere.