Which of the following defines fuel-oil?

Fuel-oil is specifically defined as hydrocarbon oils that possess a flashpoint not lower than 100 ̊F. This definition is critical as the flashpoint indicates the temperature at which the fuel can vaporize to form an ignitable mixture in air. This property is essential for safety and handling purposes, as fuels with higher flashpoints are generally considered less hazardous when it comes to combustion risk. The flashpoint is also a regulatory consideration for storage and transportation of the fuel, impacting how it is classified and utilized in heating and energy systems.

The other options do not accurately reflect the definition of fuel-oil: a mix of organic compounds might describe various materials but does not specifically delineate fuel-oil; intentionally enriched petroleum does not correspond to the classification or characteristics of fuel-oil; and a type of natural gas does not pertain to the classification of fuels derived from crude oil, which fuel-oil is. Therefore, identifying fuel-oil based on its flashpoint aligns with the industry's standards and definitions.