The process in which nitrogen is converted into a usable form for life is called
a.
nitrogen fixation
b.
photosynthesis
c.
greenhouse effect
d.
photolysis

a. nitrogen fixation

Explanation:

Nitrogen fixation is a process by which the atmospheric nitrogen is trapped by the soil bacteria which then converted into ammonia, nitrates, nitrites and other related nitrogen compounds absorbed by the plants from the soil. Nitrogen is also fixed in the soil by the lightning in which the nitric oxide and ultraviolet rays fix significant amount of nitrogen in the usable forms like ammonia, nitrites and nitrates. The soil bacteria provides these use able nitrogen forms to plants necessary for their growth and in turn receives nourishment from them.

your answer is "the 1950's"