The correct answer is A) 5% or less. The US Coast Guard regulations for inert gas systems in cargo tanks require that the oxygen content of the inert gas or gas mixture supplied to the cargo tanks must be 5% or less by volume. This ensures that the atmosphere in the cargo tanks is effectively made inert, with a low enough oxygen concentration to prevent the risk of explosions or fires. The other options are incorrect because 10% (B), 20% (C), and 15% (D) oxygen content would not be low enough to effectively inert the cargo tank atmosphere according to the Coast Guard regulations.