Which of the following is a recommended method for preventing Class B fires?

The recommendation to not refuel in confined spaces is critical for preventing Class B fires, which involve flammable liquids such as gasoline, oil, and solvents. Confined spaces often lack proper ventilation, which increases the risk of vapor accumulation. Flammable vapors can ignite easily, particularly if there is a spark or flame nearby. By avoiding refueling in these areas, the likelihood of creating a hazardous environment is significantly reduced. This practice aligns with safety standards that promote the proper handling and storage of flammable materials to prevent fire hazards.

In contrast, the other choices advocate practices that increase fire risks, such as keeping flammable liquids in open containers, which can allow vapors to escape and ignite, and refueling machinery while it's running, potentially creating sparks. Storing flammable materials near heat sources also poses a major risk, as it elevates the chance of ignition. Thus, the correct answer underscores a fundamental safety guideline to minimize fire risks associated with flammable liquids.