In a population, a shift in the average population phenotype toward the mean and away from the physical extremes. this process lowers phenotypic variety; the population is more homogeneous.

The correct answer would be stabilizing selection.

The stabilizing selection is a type of natural selection in which the phenotype towards mean value get stabilizes instead of extreme values.

It uses negative selection to remove the extreme values of the phenotype.

For example, in humans, the intermediate birth weight of babies is selected by stabilizing selection. The extreme weights (whether minimum or maximum) reduces the chances of survival of the offspring.

Similarly, plants with intermediate height tend to survive the most. Plants with short height do not receive enough sunlight and plants which are extremely tall are vulnerable to winds.

explanation:

b,bacteria convert nitrates

into nitrogen gas.

d,plants and animals take in

and use nitrogen compounds.

e,bacteria break down waste    

products and return nitrogen  

to the soil.

a,bacteria convert atmospheric

nitrogen to ammonia.

c,bacteria convert ammonia

to nitrites and nitrates.