Sulphur oxides are undesirable in flue gas because they:

Sulphur oxides (SOx), which primarily include sulfur dioxide (SO2) and sulfur trioxide (SO3), are significant pollutants released during the combustion of fossil fuels that contain sulfur, such as coal and oil. The reason these compounds are undesirable in flue gas is primarily due to their role in contributing to acid rain.

When sulphur oxides are emitted into the atmosphere, they can react with water vapor, oxygen, and other trace chemicals to form sulfuric acid (H2SO4) and related compounds. This acidic runoff can significantly impact the environment by damaging ecosystems, harming wildlife, and leading to the deterioration of buildings and monuments. These environmental effects have led to regulatory measures to limit SOx emissions from power plants and industrial sources.

The other options, while related to combustion and its efficiency, do not capture the primary environmental impact of sulphur oxides. Therefore, the contribution of sulphur oxides to acid rain is what makes their presence in flue gas particularly detrimental.