In the context of biochemical reactions, it is primarily the C) Reactants that are always changed when they participate in a reaction. Reactants are the initial substances that undergo a chemical change to form products.
Enzymes, on the other hand, act as catalysts that facilitate these reactions without being permanently altered themselves. They lower the activation energy required for the reaction to proceed but do not change in the process. Similarly, phosphate groups can be involved in various reactions (such as phosphorylation), but they also do not inherently change with each reaction they participate in.
In summary, while enzymes and phosphate groups play crucial roles in reactions, it is the reactants that undergo the actual transformation during the course of reaction.