Explanation: Not all DSC units store all transmitted and and received DSC calls in a single, undifferentiated directory. DSC (Digital Selective Calling) units vary widely in their features and sophistication. Many units, especially more advanced ones, will separate transmitted call logs from received call logs, and may further categorize received calls (e.g., distress, urgency, routine). Therefore, the statement that *all* units store them in a single directory is incorrect. Options B and C accurately describe features found in many DSC units. Having separate transmit and receive directories, or different directories for received calls sorted by priority (like distress, safety, or routine calls), is common practice to help operators manage and prioritize communications effectively. Option D reflects good operating practice; retaining DSC call logs is important for maritime logkeeping requirements, inspections, and incident investigations, even if a unit technically allows deletion. These logs often serve as a vital record of communications.