The correct answer is B) testing at least every 48 hours. According to the regulations, all required GMDSS equipment (other than survival craft equipment) must be tested at least every 48 hours to ensure it is operational. This is a requirement specified in the International Convention for the Safety of Life at Sea (SOLAS) and is necessary to maintain the reliability and readiness of the vessel's emergency communication systems. The other options are incorrect - daily testing (A) is more frequent than required, while operational use of the equipment (C) does not constitute a formal test to verify functionality. Option D is also incorrect, as the correct answer is specifically testing at least every 48 hours, not either daily testing or operational use.