Explanation: Simplex frequencies involve transmitting and receiving on the same frequency, enabling direct, point-to-point communication without the use of a repeater. This mode is efficient and widely adopted for localized, direct interactions. * **A) Normally used between ship stations and private coast stations:** Private coast stations, such as those operated by marinas, yacht clubs, or fishing companies, frequently use simplex channels for direct, short-range communications with specific vessels. This allows for straightforward coordination like docking instructions or operational messages. * **C) Normally used between ship stations:** Ship-to-ship communication is a cornerstone of marine radio, primarily utilizing simplex channels (e.g., VHF Channel 16 for hailing, or other working channels) for purposes like collision avoidance, hailing other vessels, or coordinating activities directly between ships. **B) Normally used with public coast stations:** Public coast stations (e.g., those providing general public correspondence or weather broadcasts) often utilize duplex frequencies. Duplex allows for simultaneous two-way conversations, much like a telephone call, or involves more complex repeater systems, which differs from the direct, same-frequency operation characteristic of simplex. Since both A and C accurately describe normal applications of simplex frequencies in marine communications, option D is the correct choice.