The enzyme activity can be completely stopped by

Options

A. Removing its product as fast as it is formed
B. Doubling its concentration
C. Decreasing its concentration
D. Blocking its active site

Answer

To solve the question regarding what can completely stop enzyme activity, we can analyze the options provided step by step.

### Step-by-Step Solution:

1. **Understanding Enzyme Activity**: Enzymes are biological catalysts that speed up chemical reactions by binding to substrates at their active sites. When a substrate binds to an enzyme, it forms a product, which is then released.

**Hint**: Remember that enzymes work by binding substrates at their active sites.

2. **Evaluating the Options**:
- **Option 1: Removing its product as far as it is formed**: Removing the product does not stop the enzyme activity. In fact, it can help maintain the reaction because the substrate can continue to bind to the active site.

**Hint**: Consider how product removal affects the equilibrium of the reaction.

- **Option 2: Doubling its concentration**: Increasing the substrate concentration generally increases the rate of reaction, as there are more substrates available to bind to the enzyme.

**Hint**: Think about how substrate concentration influences enzyme activity.

- **Option 3: Decreasing its concentration**: Reducing the concentration of the substrate will decrease the enzyme activity, but it will not completely stop it unless there are no substrates left to bind.

**Hint**: Reflect on what happens when there are fewer substrates available.

- **Option 4: Blocking its active site**: If an inhibitor binds to the active site of the enzyme, it prevents the substrate from binding. This means that no product can be formed, effectively stopping the enzyme's activity.

**Hint**: Consider the role of inhibitors in enzyme function.

3. **Conclusion**: The only option that can completely stop enzyme activity is **blocking its active site**. When the active site is blocked, the substrate cannot bind, and thus, no reaction occurs.

### Final Answer:
The enzyme activity can be completely stopped by **blocking its active site**.