Final answer:
Nitrogen must be fixed for plants and animals to use it. Nitrogen fixation is the process of transforming gaseous nitrogen into more usable forms such as ammonia. This is achieved through biological, physical, or chemical processes, with soil bacteria and legume plants being a key source of biological nitrogen fixation. The correct option is 4.
Step-by-step explanation:
In order for nitrogen to be used by most plants and all animals, it must be fixed.
Nitrogen fixation is the process by which gaseous nitrogen is transformed into more readily available forms such as ammonia. This process improves agricultural production as nitrogen is required by plants for nucleotide and protein formation.
Nitrogen can be fixed through biological, physical, or chemical processes. The most important source of biological nitrogen fixation is the symbiotic interaction between soil bacteria and legume plants, such as soybeans and peanuts. The correct option is 4.