The correct answer is A) when two or more stratified layers of different density LNG in the same tank suddenly mix together, causing a rapid increase in boil-off vapors. Rollover is a term used in LNG cargo operations to describe the sudden mixing of LNG layers with different densities within the same tank. This can happen when the upper layer warms up and becomes less dense than the colder, denser layer below. When the layers suddenly mix, it causes a rapid increase in boil-off vapors, which can be dangerous if not properly managed. The other answer choices are incorrect because they do not accurately describe the meaning of the term "rollover" in the context of LNG cargo operations. Option B refers to a different phenomenon, option C is about transferring LNG between tanks, and option D is about custody transfer at the terminal, which is a separate process.