6. Which of the following terms best defines the spontaneous mixing of a tank's liquid contents when a heavier layer forms above a less dense lower layer?

Rollover accurately describes the spontaneous mixing of liquid layers in a tank when a denser layer is positioned above a less dense one. This instability results in a sudden inversion and mixing, potentially increasing boil-off rate and tank pressure, unlike flash over, boil off, or cascading which refer to distinct processes.