Explanation: Receiving a Ch-70 DSC Distress alert signals an urgent maritime emergency. The initial action is to silence the audible alarm and carefully review the incoming message. This message contains vital information such as the distressed vessel's identity, position, and the nature of the emergency. According to GMDSS regulations, after reviewing the DSC alert, you must immediately set up a watch on VHF Ch-16. This is the international voice Distress, Safety, and Calling channel where the distressed vessel will transmit their voice "Mayday" communications and subsequent traffic. Options B, C, and D are incorrect. B: While informing the master is important, it's not the immediate operational radio procedure. C: GMDSS mandates active participation; waiting for RCC instructions negates the purpose of "all ships" Distress alerts. D: It is crucial *not* to immediately acknowledge a DSC Distress alert via DSC. Doing so can prevent a Rescue Coordination Center (RCC) or other more appropriate stations from acknowledging and coordinating the response, potentially causing confusion or delaying assistance. A DSC acknowledgment should only be made by an RCC or if no other station acknowledges after a reasonable period *and* you are in a position to render assistance. The immediate listening action is on Ch-16.