Blocking of enzyme action by blocking its active sites is

The correct option is "competitive inhibition".

Step-by-Step Explanation:

1. Understanding Enzymes and Active Sites:
Enzymes are biological catalysts that speed up chemical reactions. Every enzyme has a specific region called the active site. The molecule that the enzyme acts upon, known as the substrate, binds to this active site to undergo a chemical reaction.

2. What is Competitive Inhibition?
In competitive inhibition, a molecule (known as the inhibitor) resembles the substrate structurally. Because of this structural similarity, the inhibitor competes directly with the substrate for binding to the same active site on the enzyme.

3. Mechanism of Blockage:
When the competitive inhibitor binds to the active site, it physically blocks the substrate from binding. Since the active site is occupied, the enzyme's catalytic action is blocked, thereby decreasing the rate of the reaction. This inhibition can be overcome by increasing the concentration of the substrate, which outcompetes the inhibitor for the active site.

4. Distinguishing from other Options:
- Allosteric inhibition / Non-competitive inhibition: The inhibitor binds to a site other than the active site (called the allosteric site), causing a shape change in the enzyme so the substrate can no longer bind or react effectively. They do not block the active site directly by binding to it.
- Feedback inhibition: This is a regulatory mechanism where the end product of a metabolic pathway inhibits an upstream enzyme (often through allosteric regulation), rather than a specific mechanism of direct active-site blockage by structural mimics.