Understanding the Question:
The question asks which of four listed volatile oils is heavier than water.
Key Concept:
Specific gravity decides whether an oil floats or sinks in water, and while most essential oils have a specific gravity under 1 and float, a few oils with dense aromatic aldehydes are the exception.
Checking Each Option:
Cumin oil, dominated by cuminaldehyde, has a specific gravity below 1 and floats.
Fennel oil, dominated by anethole, and lemongrass oil, dominated by citral, are also both lighter than water and float on the surface.
Cinnamon bark oil is rich in cinnamaldehyde, a dense compound that pushes the oil's overall specific gravity above 1, unlike the other three.
Final Answer:
Cinnamon oil is the one among these that is heavier than water and sinks.