Final answer:
A spontaneous chemical reaction in a cell signifies that the reaction has a negative change in free energy, and thus is an exergonic reaction. Endergonic reactions, in contrast, require an input of energy and are non-spontaneous.
Step-by-step explanation:
If you observed a chemical reaction proceed spontaneously in a cell, it would be true that c. the reaction has a negative change in free energy. This is because a spontaneous reaction, by definition, occurs without an external input of energy and moves towards a lower energy state, which equates to a negative change in free energy (ΔG). Therefore, spontaneous reactions are termed exergonic reactions. These reactions release energy and are thermodynamically favorable.
In contrast, an endergonic reaction requires an input of energy, where the products have more free energy than the reactants. As such, endergonic reactions are non-spontaneous and will not occur without the addition of energy.