Concept:The process described is photosynthesis, in which green plants make food using carbon(IV) oxide, water, and light energy.Explanation:The reaction is: 6CO2+6H2OsunlightchlorophyllC6H12O6+6O2In the equation, the label I points to the condition needed to drive the reaction.Green plants cannot carry out photosynthesis without light energy.This light energy comes from the sun, so the labelled part is sunlight.Enzymes help in the process, but they are not shown as the label.Water and carbon(IV) oxide are reactants, not the external condition represented by I.Answer:C. sunlight