Nitrogen fixation
a. is the process where nitrogen is converted to a usable form
b. is accomplished by plants when they break down soil
c. occurs as a result of photosynthesis
d. none of the above

Correct A). Is the process where nitrogen is converted to a usable form  

Nitrogen fixation is the process of conversion of nitrogen gas present in the atmosphere into solid nitrogen, which is usable by the plants by the nitrogen-fixing bacteria is called as nitrogen fixation.

The nitrogen-fixing bacteria have a symbiotic relationship with the root nodules, which help them to provide nitrogen components that help them to grow.

Example: Rhizobium, Nostoc etc.