Among the options provided, grasslands that are undergoing severe drought would tend to reduce genetic variation in a population because of evolution by natural selection.
In such an environment, the drought would create selective pressures that favor individuals with specific traits better adapted to survive the dry conditions, while those without those traits may perish. This can lead to a reduction in genetic variation over time as the population becomes more homogenized in terms of the traits that are advantageous for survival in that specific environment.