Answer: A concentration ratio of 100% exclusively.

**Explanation:** - **Option A** is correct because a concentration ratio of 100% indicates that a single firm controls the entire market, which is characteristic of a monopoly. The concentration ratio measures the combined market share of the largest firms, ranging from 0% (perfect competition) to 100% (monopoly). - **Option B** is incorrect because a Herfindahl-Hirschman index (HHI) of zero implies perfect competition, not a monopoly. The HHI is calculated by squaring the market shares of all firms and summing them. In a monopoly, the HHI would be 1 (or 10,000 if expressed as a percentage). - **Option C** is incorrect because a monopoly is indicated by a concentration ratio of 100%, not an HHI of zero. The HHI of zero contradicts the definition of a monopoly.

Author: LeetQuiz Editorial Team