What do the acronyms UFL, UEL, LFL, and LEL stand for?

The selected answer is accurate because it identifies each term correctly within the context of flammability and explosiveness of materials. Specifically, the term "Upper Flammable Limit" (UFL) refers to the maximum concentration of vapor in the air that can ignite; beyond this limit, the mixture is too rich to burn. The "Upper Explosive Limit" (UEL) represents the highest concentration of a gas or vapor in air that can ignite and cause an explosion under specific conditions.

Conversely, "Lower Flammable Limit" (LFL) and "Lower Explosive Limit" (LEL) denote the minimum concentrations of vapor needed in air to ignite and cause combustion or explosion, respectively. Understanding these definitions is crucial for safety in handling volatile substances and for preventing fire and explosion hazards in various industries.

The other options misinterpret or inaccurately define the terms, failing to align with accepted safety standards and terminology used in the field of industrial safety.