Which of the following is a characteristic of vapors?

A defining characteristic of vapors is that they are invisible gases that arise from liquids. This state occurs when a liquid reaches a temperature where it starts to evaporate or boil, transitioning into a gaseous form. Vapors can exist even at temperatures below the boiling point of the corresponding liquid, making them a common aspect of various substances.

The other choices have limitations. Vapors are not exclusively formed under high pressure; instead, they can form at various pressures depending on the temperature and the properties of the liquid. While vapors will indeed condense back into liquid upon cooling, this characteristic is more a property of all gases rather than a defining trait of vapors specifically. Lastly, while it might be true that vapors can have a lower density than their liquid counterparts under certain conditions, it is not universally applicable, as the density can vary based on temperature and the specific liquid involved. Thus, invisibility and the gaseous state originating from liquids are fundamental to understanding what vapors are.