'Adjective' is a word which qualifies/ modifies/describes a noun'.
All the given options are adjectives which can qualify one or the other noun but the adjective which describes the given noun 'phenomenon' is Option B - 'natural' because 'natural' means 'something which occurs by itself in nature', and formation of soil is mentioned here.
Option A - 'common' is an adjective which refers to something usual, general or known thing, Option C - 'universal' is an adjective which refers to something which is applicable in a similar way around the world/universe, Option D - 'nuclear' is an adjective which refers to something which is related to the nucleus / centre / main part of something.
Therefore, the correct option B - 'natural' is the correct adjective which completes the given sentence.
The completed sentence thus becomes: The formation of soil is a natural phenomenon.