Explanation: INMARSAT mini-M is a satellite communication service designed for mobile users. The "mini-M" designation refers to a generation of more compact terminals and services that specifically utilize **spot beams**. Spot beams are narrow, high-power radio frequency beams transmitted from a geostationary satellite, covering a smaller geographical area with greater signal strength. This allows for the use of smaller, less powerful, and more affordable antenna terminals, making the system practical and accessible for **small craft communications**, as well as land mobile users. Option A and C are incorrect because SONAR and depth finders are acoustic systems that use sound waves, not radio waves or satellites. Option B is partially true as INMARSAT is a global marine satellite system, but option D provides a more specific and technically accurate description, highlighting the key feature of "mini-M" with its use of spot beams for smaller terminals and craft.