The best description of what will happen to the color variation of the ground beetles after several years in an environment with dark soil is that the beetles' color will not change. This is because individual beetles do not change their color in response to their environment; rather, natural selection may favor the survival of darker beetles over time. As a result, the population may gradually shift towards a higher proportion of dark brown beetles, but the actual color of individual beetles in the next generation will still vary.
So while the overall population may become darker over time due to selective pressure, the beetles' color itself will not change. Thus, the best option reflects the population dynamics rather than an immediate change in color for all individuals.