The process of covering nitrogen to a form plants can use

The step in the nitrogen cycle that takes the nitrogen from the air and converts it to ammonia (the form that can be used by plants) is called Nitrogen Fixation.

the correct answer is option d.

an appositive is a noun, noun phrase or noun clause which follows another noun or noun phrase in order to describe it or name it another way. they are usually used between commas, brackets or dashes.

the example sentence with the correct use of commas is the following:

her uncle, the man over there in the red shirt, owns the hardware store in town.