The correct answer is C) 30% of the lower flammable limit. According to the regulations for LNG (liquefied natural gas) ships, the fixed gas detection system must activate an audible and visual alarm when the gas concentration reaches 30% of the lower flammable limit (LFL). This means that the alarm will be triggered when the gas concentration is 30% of the minimum concentration required for the gas to ignite and burn. The lower flammable limit is a critical safety threshold, and the 30% level provides an early warning to the crew to take appropriate action before the gas concentration becomes dangerously high. The other options are incorrect because they do not align with the specific regulatory requirement for LNG ships. Option A (30% by volume) and Option B (20% by volume) do not reference the lower flammable limit, which is the critical measure for gas detection systems on these vessels. Option D (20% of the lower flammable limit) is also incorrect, as the regulatory requirement is 30% of the LFL.