What change occurred after horses were removed from a fenced area, leading to less soil erosion on rainy days?

The correct choice highlights the impact of plant growth on soil stability. When horses are removed from a fenced area, the absence of grazing pressure allows grass and other vegetation to rejuvenate and grow. This plant growth is crucial because grasses have extensive root systems that anchor the soil in place.

During rainy days, the roots of these plants help hold the soil together, reducing the amount of loose soil that can be washed away with rainfall. As a result, the reestablishment of grass contributes to enhanced soil retention and minimizes erosion, which is particularly important in preventing water runoff that can lead to further soil degradation.

In contrast, the other options do not accurately reflect the ecological processes at play. The idea that the fence contained the soil does not address the importance of plant growth in preventing erosion. The assertion that the soil was completely gone is inaccurate, as the focus is on how the soil is preserved rather than lost. Lastly, a decrease in the amount of rain would not directly relate to the effects of horse removal; rather, it is the vegetation response that mitigates erosion during existing rainfall.