The process of natural selection is based on the assumption that:
(1) environmental changes will cause changes in body structure in individuals
(2) most changes from generation to generation are the result of mutations
(3) part of the population of organisms always remains stable
(4) different traits inherited by offspring have different survival value

Option B, most changes from generation to generation are the result of mutations

Explanation:

Through the process of evolution, an individual changes from one generation to other generation. These variations are caused by genetic changes.

Genetic variations are caused by -

a) Genetic mutation

b) genetic recombination

Genetic variation leads to alteration of gene or protein function due to which differences traits arises in an organisms. Thus , it can be said that genetic mutation brings variation in structure and thus leads to evolution of any species.

As long as  generations of individuals with the newly developed trait continues to mate and reproduce, new trait becomes  common in a population.

Hence, option B is correct