When a drop of dilute nitric acid is added to water surrounding a hydra, what is the action of the acid?

The addition of dilute nitric acid to the water surrounding a hydra serves as a stimulus. In biological terms, a stimulus refers to any change in an organism's environment that can provoke a response. In this case, the diluting effect of nitric acid alters the chemical environment of the water, which can trigger a reaction from the hydra. This change can lead to various physiological responses, such as movement or retraction as the organism reacts to the introduced stimulus.

Understanding the nature of how organisms interact with their environment is essential in biology. Stimuli such as chemicals, light, or temperature changes can provoke various responses from living creatures based on their biological mechanisms. While the other options relate to different concepts—like a response being the actual action taken by the organism after sensing the stimulus—recognizing the role of the acid as a stimulus is key in understanding the initial trigger for the hydra's reaction.