C. Enzymes work only at their optimum temperature and pH

Enzymes are biological catalysts that speed up chemical reactions in living organisms. They are highly specific and efficient in their action. The correct statement about enzymes is that they work only at their optimum temperature and pH.

1. Enzymes and Activation Energy:
- Enzymes do not increase the activation energy of a reaction (Option A is incorrect). In fact, enzymes lower the activation energy required for a reaction to occur, thus accelerating the reaction.
- Activation energy is the energy needed to break the existing bonds in reactant molecules and initiate a chemical reaction. Enzymes accomplish this by providing an alternative reaction pathway, which requires less energy.

2. Enzyme-Substrate Interaction:
- Enzymes interact with specific molecules called substrates, and the region on the enzyme where the substrate binds is called the active site.
- Enzymes are not used in excess compared to the reagent (Option B is incorrect). Enzymes are highly efficient and work in small amounts. They can catalyze multiple reactions before being used up or denatured.

3. Optimum Temperature and pH:
- Enzymes have specific conditions under which they function optimally. This includes their temperature and pH requirements.
- Enzymes work only at their optimum temperature and pH (Option C is correct). Deviating from these optimal conditions can negatively affect enzyme activity.
- Temperature: Enzymes have an optimal temperature at which they work most effectively. Increasing the temperature beyond this point can denature the enzyme, altering its active site and reducing its catalytic activity. Similarly, decreasing the temperature can slow down enzyme activity.
- pH: Enzymes also have an optimal pH range. Changes in pH can alter the charge distribution within the enzyme and affect its three-dimensional structure. This can lead to a loss of enzyme activity or denaturation.

4. Enzyme Inhibition:
- The activity of enzymes can be affected by other compounds (Option D is incorrect). Inhibitors can bind to the active site or other regions of the enzyme, reducing or completely blocking its catalytic activity. This can be reversible or irreversible depending on the type of inhibitor.

In conclusion, enzymes work optimally within specific temperature and pH ranges, and deviations from these conditions can affect their activity. Enzymes lower the activation energy of a reaction, and they do not need to be used in excess compared to the reagent. Additionally, enzyme activity can be affected by other compounds through inhibition.