Explanation:
Acid rain is formed when sulfur dioxide (SO2) and nitrogen oxides (NOx) gases react with water, oxygen, and other chemicals in the atmosphere to form sulfuric acid (H2SO4) and nitric acid (HNO3). These acids can then fall to the ground as rain, snow, or dry particles.

Hydronium ions:
When sulfuric acid or nitric acid dissolves in water, they dissociate to release hydrogen ions (H+). These hydrogen ions can then react with water molecules to form hydronium ions (H3O+).

Mobilization of toxins:
Hydronium ions are highly reactive and can easily react with various substances present in the environment, including metals such as iron (Fe), magnesium (Mg), and aluminum (Al). This reaction between hydronium ions and metals leads to the mobilization of these toxins.

Iron (Fe):
Hydronium ions can react with iron present in soil or other materials, leading to the release of soluble iron ions (Fe2+ or Fe3+). These soluble iron ions can then be transported through the soil or water, potentially causing harm to aquatic organisms or affecting the quality of drinking water.

Magnesium (Mg):
Similarly, hydronium ions can react with magnesium present in soil or other materials, releasing soluble magnesium ions (Mg2+). This can also lead to the mobilization of magnesium and its subsequent transport through the environment.

Aluminum (Al):
Hydronium ions can have a particularly strong effect on aluminum. Aluminum is naturally present in soil and rocks, but under normal conditions, it exists as insoluble aluminum hydroxide or aluminum oxide. However, in the presence of hydronium ions, aluminum can undergo dissolution, leading to the release of soluble aluminum ions (Al3+). These soluble aluminum ions can be highly toxic to aquatic organisms and can also contribute to the acidification of soils.

Conclusion:
In summary, the hydronium ions formed during the dissolution of sulfuric acid and nitric acid in acid rain can mobilize toxins such as iron, magnesium, and aluminum. This mobilization can have harmful effects on the environment, including the contamination of water sources and the deterioration of soil quality. Therefore, option 'C' (Aluminium) is the correct answer.