**Core Concept:**
A competitive inhibitor is a substance that competes with the substrate for binding to an enzyme. Inhibitors can be classified into three types: competitive, uncompetitive, and non-competitive inhibitors. Competitive inhibitors bind to the active site of an enzyme, blocking substrate binding and reducing the rate of the enzyme's reaction. Conversely, uncompetitive inhibitors bind to the enzyme-substrate complex, while non-competitive inhibitors bind to the enzyme's allosteric site, altering the enzyme's conformation and reducing its catalytic activity.

**Why the Correct Answer is Right:**
A competitive inhibitor (Option D) will decrease the enzyme's activity by competing with the substrate for binding to the enzyme's active site. This results in a decreased rate of the enzyme's reaction, as the inhibitor prevents substrate binding and the enzyme's catalytic function.

**Why Each Wrong Option is Incorrect:**
A non-competitive inhibitor (Option C) binds to the enzyme's allosteric site, altering the enzyme's conformation and reducing its catalytic activity. This is different from a competitive inhibitor, which affects the active site.

An uncompetitive inhibitor (Option B) binds to the enzyme-substrate complex, which is distinct from the competitive inhibition where the inhibitor competes with the substrate for the active site.

An uncompetitive inhibitor does not affect the enzyme's activity when substrate is present (Option A), unlike a competitive inhibitor which decreases the enzyme's activity by competing with the substrate for the active site.

**Clinical Pearl:**
Understanding the differences between competitive, uncompetitive, and non-competitive inhibitors is crucial for pharmacology and drug therapy. These classifications help in predicting the effects of inhibitors on enzymes and designing effective drug therapies targeting specific enzymes.

**Correct Answer:** Competitive inhibitor (Option D) - A competitive inhibitor will decrease the enzyme's activity by competing with the substrate for binding to the enzyme's active site.