Step 1: Understanding Stabilizing selection.
Stabilizing selection occurs when extreme values of a trait are selected against, and intermediate values are favored. In this case, both very large and very small horn lengths are disadvantageous, and beetles with moderate horn lengths have a reproductive advantage.
Step 2: Explanation of the other options.
Directional selection favors extreme traits in one direction.
Disruptive selection favors extreme traits at both ends.
Artificial selection is driven by human intervention.