After which event could you say that evolution has occurred?
a. an animal gets fat
b. a white-eyed fly is born into a population of brown-eyed flies
c. an allele frequency changes in a population
d. a new predator comes into an environment willｇｉｖｅ ｍｅｄａｌ

 C, because alleles are alternative forms of a gene that arise from mutation.

C. An allele frequency changes in a population

Explanation:

Evolution refers to the accumulation of genetic variations in the gene pool of population leading to the origin of the better adapted population. Evolutionary agents such as mutations, natural selection, gene flow, etc. change the allele frequency in the gene pool of the population.  

The changed allele frequency of a population represents that fact that one or more of the above mentioned evolutionary agents are working on the gene pool and population has occurred.  

Solar energy drives the cycle by evaporating water from the oceans, lakes, rivers, and even the soil. Other water moves from plants to the atmosphere through the process of transpiration. Plants put down roots into the soil to draw water and nutrients up into the stems and leaves. Some of this water is returned to the air by transpiration (when combined with evaporation, the total process is known as evapotranspiration).