Whether a volatile oil floats or sinks in water depends entirely on its specific gravity compared with water, which is taken as 1. Most volatile oils used in pharmacy, including cumin, fennel, and lemongrass, have a specific gravity somewhat below 1, so when shaken with water and left to stand, they always separate out as the upper layer.
Only a small number of oils are dense enough to break this pattern. Cinnamon bark oil is one of them, its high cinnamaldehyde content pushes its specific gravity above 1, typically in the range of about 1.03 to 1.05, so instead of floating it drops through the water and collects as the lower layer. Clove oil and cassia oil behave the same way for the same reason, a heavy aromatic aldehyde or phenolic constituent.
Since cumin, fennel, and lemongrass all stay lighter than water while cinnamon does not, cinnamon is the oil that is heavier than water and the correct answer here.