The correct answer is option 4: Through the activity of bacteria. Bacteria play a crucial role in making nitrogen available for plant growth in nature. Nitrogen gas makes up approximately 78% of the Earth`s atmosphere, but plants cannot directly use this form of nitrogen. Bacteria known as nitrogen-fixing bacteria have the unique ability to convert atmospheric nitrogen into a form that plants can use, called ammonium. These bacteria possess an enzyme called nitrogenase, which allows them to convert atmospheric nitrogen gas into ammonium.

This process, known as nitrogen fixation, occurs in the soil. The bacteria colonize the roots of certain plants, such as legumes (e.g., beans, peas, and clover), forming nodules. Inside these nodules, the bacteria convert atmospheric nitrogen into ammonium, which can then be absorbed by the plant roots. Some free-living bacteria in the soil also contribute to nitrogen fixation.

Once the ammonium is inside the plant, it is used to produce amino acids, proteins, and other nitrogen-containing compounds essential for plant growth and development. Additionally, other groups of bacteria in the soil, known as nitrifying bacteria, further convert ammonium into nitrates, which are also readily absorbed by plants.

Therefore, through the activity