Which of the following best represents a chemical response to external stimuli in plants?

Chemotropism is indeed the best representation of a chemical response to external stimuli in plants. This phenomenon occurs when a plant's growth responds to chemical gradients in its environment. In response to certain chemicals, plants can alter their growth direction, often to seek out nutrients or to avoid harmful substances. For example, plant roots may grow toward higher concentrations of nutrients, demonstrating a direct chemical response to their environment.

In contrast, courtship behavior pertains to animal reproductive strategies and does not involve plant responses. Hydrotropism, while it involves growth towards moisture, is specifically a response to water rather than a chemical stimulus. Arrhythmic refers to a lack of regularity or rhythm, which is not relevant to plant responses to stimuli. Thus, chemotropism stands out as the primary example of how plants can alter their growth in reaction to chemical stimuli in their surroundings.