Which fuel-oil is considered the heaviest commercially available fuel-oil?

No. 6 fuel-oil is recognized as the heaviest commercially available fuel-oil. It is a high-viscosity oil that requires preheating before it can be efficiently burned in most combustion systems. This fuel is typically used in large industrial applications, marine shipping, and power generation, where its density and energy content can be utilized effectively, despite the need for specialized equipment to handle its viscosity.

In contrast, the other options represent lighter grades of fuel oil. For example, No. 2 fuel-oil is a common residential and commercial heating oil, while No. 1 fuel-oil is even lighter and primarily used in applications requiring low viscosity. No. 4 fuel-oil falls in between these two, but it does not reach the density or viscosity levels of No. 6 fuel-oil. This distinction makes No. 6 fuel-oil the heaviest among commercially available fuels.