Final answer:
The term for an organism's ability to respond to external stimulation is 'irritability,' which exemplifies the broader concept of responsiveness to environmental changes. Option 2) irritability is the correct answer.
Step-by-step explanation:
The ability, possessed by all organisms, to respond to external stimulation is known as irritability. The correct answer is irritability.
Irritability refers to the immediate sensitivity and response to external and internal stimuli. Living organisms exhibit responsiveness as they can adjust to changes in their environments. This is seen when organisms move towards sources of food or water and away from dangers, or respond to changes in their internal environment, like increased body temperature leading to sweating. An example of a plant's response is phototropism, which is the movement towards light.
Even single-celled organisms like bacteria display this characteristic through processes like chemotaxis or phototaxis, where they move towards or away from chemical substances or light, respectively.