Answer: Strong-form efficient.

**Explanation:** - **Option A (Correct):** In a strong-form efficient market, all information, including private or insider information, is fully reflected in market prices. Therefore, insiders cannot earn abnormal returns by trading on private information. - **Option B (Incorrect):** In a weak-form efficient market, prices only reflect historical trading data (e.g., past prices and volumes). Private information is not incorporated, allowing insiders to potentially earn abnormal profits. - **Option C (Incorrect):** In a semi-strong-form efficient market, prices reflect all publicly available information but not private information. Thus, insiders could still exploit private information for abnormal returns.

Author: LeetQuiz Editorial Team