The correct answer is option 4, bioremediation. Bioremediation refers to the process of using microorganisms, such as bacteria or fungi, to treat areas of land or sea that have been contaminated by pesticides, oil, or solvents. These microorganisms have the ability to break down or transform the harmful pollutants into less harmful substances or remove them from the environment completely.

Option 1, eutrophication, is not the correct answer. Eutrophication is the excessive growth of algae and plants due to an increase in nutrients in a body of water, leading to a decrease in oxygen levels and harmful effects on aquatic life.

Option 2, nitrification, is also not the correct answer. Nitrification is the conversion of ammonia into nitrates by bacteria in the soil or water. It is a natural process in the nitrogen cycle.

Option 3, ammonification, is not the correct answer either. Ammonification is the conversion of organic nitrogen compounds into ammonia by bacteria during the decomposition of organic matter.

In summary, bioremediation is the process of using microbes to treat areas contaminated with pollutants, while options 1, 2, and 3 are not related to this specific process of pollution cleanup.