Answer: Maintaining inadequately diversified portfolios.

**Explanation:** - **Option A (Correct):** Overconfidence bias leads investors to overestimate their knowledge or abilities, often resulting in poorly diversified portfolios. This lack of diversification exposes them to significant downside risk. - **Option B (Incorrect):** This behavior is associated with **endowment bias**, where investors overvalue assets they already own due to familiarity, not overconfidence. - **Option C (Incorrect):** This is a manifestation of **loss-aversion bias**, where investors hold onto losing investments hoping to break even, despite fundamental analysis suggesting otherwise. *Reference: Portfolio Management - Behavioral Biases in Financial Decision Making.*

Author: LeetQuiz Editorial Team