A herbicide that causes abnormal plant tissue development is known as a _________.

A herbicide that causes abnormal plant tissue development is classified as a plant growth regulator. These substances can significantly alter plant processes, influencing growth, morphology, and development in ways that may appear abnormal.

Plant growth regulators include a range of chemicals that manipulate the hormonal balance in plants, leading to effects such as stunted growth, overgrowth in certain tissues, or abnormal leaf and stem structure. This is particularly relevant in aquatic pest control, as some plants may be controlled effectively by inducing these changes, ultimately leading to the death of the undesirable aquatic weeds or allowing for better management of the ecosystem.

In contrast, the other options represent different concepts. "Plant pest" refers to organisms causing damage to plants, which is not directly related to herbicide action. "Weed killer" is a general term for any herbicide but does not specifically capture the mechanism of inducing abnormal tissue development. "Herbicide inhibitor" suggests a chemical that prevents herbicide activity, which is opposite to what is being described in the question. Thus, the classification as a plant growth regulator accurately summarizes the role of the herbicide in causing abnormal development.