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?

Correct Answer: Option D — rollover

Rollover occurs in liquefied gas or stratified crude oil tanks when lighter liquid evaporates at the surface, increasing density until a heavier layer sits atop a lighter layer. This unstable density gradient triggers rapid, spontaneous convective mixing and sudden high-rate boil-off.