Which of the following factors can affect the integrity of a UST?

The integrity of a UST (Underground Storage Tank) can be significantly influenced by a range of factors, which is why the choice highlighting soil conditions, temperature changes, and tank age is correct.

Soil conditions play a critical role because the type of soil surrounding the tank can affect its stability and how it interacts with environmental factors. For example, aggressive soils may cause corrosion if the tank is not adequately protected or constructed from suitable materials.

Temperature changes can lead to physical expansion or contraction of the tank and the fuel it contains. Fluctuations in temperature can also contribute to condensation within the tank, potentially leading to water accumulation that can affect the stored fuel's quality and the tank's integrity.

Tank age is another important factor; older tanks may be more susceptible to wear, corrosion, and structural failures due to prolonged exposure to elements and chemicals over time. Regular inspections and maintenance become increasingly important as tanks age to ensure their integrity and performance remain intact.

The other options, while they may have some impact on UST operations, do not encompass the broader range of physical and environmental influences that can threaten the structural integrity of the tank itself as thoroughly as the chosen option does.